Exact tidal study extends back to a fourteenth century record containing sexagesimal times. Earlier commentators have taken these times to be high water. This study shows they are for the later moment of when the flood ends; and, that eighteenth and nineteenth century elevation measurements formed a completely different way of considering the tide, leading directly to reliable predictions. The result shows the Thames tide to have been more consistent than previously stated; and consequently the actual change more accelerated. Therefore tidal current predictions are shown to have a longer history than have water surface elevation levels.

Receiver Autonomous Integrity Monitoring (RAIM) is a method implemented within the receiver to protect users against satellite navigation system failures. For a receiver to execute a RAIM calculation, two conditions must be met: a minimum number of satellites and adequate satellite geometry. The non-existence of the minimum number of satellites (five) is referred to as a RAIM hole. Current regional and global RAIM availability studies use spatial (grid-based) and temporal sampling intervals driven by a trade-off between accuracy and computation workload. The implication of minimising computational load is that accuracy is compromised and potential RAIM holes remain un-sampled, with potential risk to safety. This paper proposes a direct and computationally efficient method (as opposed to the grid-based search approach) to predict RAIM holes. The method is based on the precise computation of satellite coverage (footprint) boundaries, the intersection points and analysis of the topology of the regions of intersection. Test results show that the proposed method is highly accurate and requires minimal computational load compared to the current approach.

This paper details the current status of the development of an ‘automatic’ low-cost system based on wireless communications technology to provide continuous tracking of the location of devices in all environments. This task requires a multi-disciplinary approach combining communications systems design, digital signal processing to extract ranges and, importantly, approaches from the field of geodesy to develop novel network positioning techniques for ad-hoc networks. Such a network will support a number of services relevant to crime management where seamless tracking is required. The paper discusses the process for developing the system, christened intelligent pervasive location tracking (iPLOT), with a particular reference to user and system requirements, and how these have been used to explore a network positioning strategy.

The instructor of a full mission marine simulator faces a daunting task, controlling several target ships under varying environmental conditions while responding to a variety of communications. This project applies computer agent technology to an instructor station. Each target ship is controlled by a collision avoidance agent which takes command of the ship and a track-keeping agent which acts as an assistant, sending the collision avoidance agent advisory messages. Experiments have been performed for a number of collision situations in varying environmental conditions. An analysis of the results demonstrates the potential of such a system for producing realistic target ship motion, including track-keeping and some collision avoidance manoeuvres, thus reducing the instructor’s immediate load.

Small Unmanned Aerial Vehicles (UAVs) or inexpensive airplanes, such as a Cessna single engine aircraft, require a navigation system with a cheap, compact and precise sensor. Over the past ten years, GPS receivers have begun to be used as primary or alternative navigation sensors, because their use can significantly reduce the overall system cost. This paper describes a navigation system incorporating a velocity-based attitude estimation system with an attitude determination system using multiple antennae, which was implemented and tested using a UAV. The main objective was to obtain precise attitude information using low cost GPS OEM boards and antennae. Attitude boundaries are derived from the relationship between the body frame and the wind coordinates, which are used to validate the resolved cycle ambiguity in an Euler angle domain. Angular rate based on Doppler measurements was used to exclude the degenerate pseudo-roll angle information during severe uncoordinated flight. Searching for cycle ambiguity at every epoch of the flight showed that the developed system gave reliable cycle integer solutions, although the carrier phase measurement was subject to additional errors, such as multipath, external interference, and phase centre variation. A flight test was performed using a 1/4-scale Piper J3 Cub model, CMC Allstar OEM boards, OEM AT575-70 antennae, and 700 MHz PC104 board.

The upcoming GNSS Galileo, with its new satellite geometry and frequency plan, will not only bring many benefits for navigation and positioning but also help to improve ionosphere delay estimation. This paper investigates ionosphere estimation with Galileo and compares it with the results from GPS-only and combined GPS-Galileo. The standard deviation of the Vertical Total Electron Content (VTEC) at a certain location can improve significantly by 40 per cent. Various Galileo configurations are considered to assess the differences in frequency plan and signals to be used. The IGS network, which is involved in producing the current IGS Global Ionosphere Map (GIM) is used in the simulation for more realistic results. A more accurate GIM will in its turn improve navigation and positioning performance.

The paper introduces a new measure of collision risk derived from the concept of a ship domain. Its simplicity and the fact that it takes into account the courses of both ships, makes it a good alternative to the commonly used term of the distance at the closest point of approach (DCPA). The measure being presented is flexible enough to be applied in combination with any given ship domain. Derivations of all the necessary equations for the Fuji domain have been presented in detail. Additionally, the paper contains numerical algorithms that are capable of determining value of the measure for any other ship domain. Based on these algorithms a generic method for determination of the necessary course alteration is developed. Also, an example of an already known formula for risk assessment, that may benefit from the measure, is provided. All of the algorithms, formulas and their derivations in the text are presented explicitly, so that they could be directly applied in any collision avoidance or VTS system.

In this paper we present a micro-electrical mechanical system (MEMS) based pedestrian navigation system (PNS) for seamless positioning. The sub-algorithms for the PNS are developed and the positioning performance is enhanced using the modified receding horizon Kalman finite impulse response filter (MRHKF). The PNS consists of a biaxial accelerometer and a biaxial magnetic compass mounted on a shoe. The PNS detects a step using a novel technique during the stance phase and simultaneously calculates walking information. Step length is estimated using a neural network whose inputs are the walking information. The azimuth is calculated using the magnetic compass, the walking information and the tilt compensation algorithm. Using the proposed sub-algorithms, seamless positioning can be accomplished. However, the magnetic compass based azimuth may have an error that varies according to the surrounding magnetic field. In this paper, the varying error is compensated using the MRHKF filter. Finally, the performance enhanced seamless positioning is achieved, and the performance is verified by experiment.

The notion of the earth as a perfect sphere has served navigators quite well over the centuries and it continues to provide a basis for instruction and practical navigation. Aside from the fact that modern navigators employ refined models of the earth such as the WGS84 ellipsoid, which together with satellite positioning gives unprecedented positioning accuracy, the question arises as to just how good are the spherical results for distance when compared to the results for distance obtained from the spheroidal model. In this document a comparison is made of great circle distance (GCD) on the sphere with great ellipse distance (GED) on the spheroid. This comparison is then repeated for rhumbline distances. In each case it is concluded that the difference in using the sphere when compared to the spheroid is near 0·5%.

Satellite velocity determination using the broadcast ephemeris is discussed and it is pointed out that the conventional rotation matrix method involves a complicated process of computation. This paper proposes an alternative method using a simple differentiator to derive satellite Earth-Centred-Earth-Fixed (ECEF) velocity from the ECEF satellite positions that are calculated using the standard ICD-GPS-200 algorithm. The proposed algorithm simplifies the velocity transformation procedure, and therefore provides a good alternative. It is demonstrated that ±1 mm/s per axis ECEF satellite velocity is achievable by using the first-order central difference of a Taylor series approximation.

A closed-form formula is also derived for the determination of GPS satellite ECEF acceleration using the broadcast ephemeris. This formula is capable of accuracies better than ±0·1 mm per second squared in each axis. With such a high accuracy of satellite acceleration in real-time, it is possible to detect the line-of-sight range acceleration precisely and as such it is concluded that a GPS receiver can be considered as a precise accelerometer.

The success of the position differential method implies that real-time satellite ECEF velocities can be directly derived through numerical differentiation of the position polynomials. This is desirable for GPS velocity determination applications which require high output rate results in real-time. The derived closed-form formula for GPS satellite ECEF acceleration would benefit those who attempt to use a GPS receiver as an accurate accelerometer in real-time.

The sight-run-sight case involves a series of two or more sights in which each sight is ‘run on’ to the time of the last sight. There are several known techniques to account for the run, which is for convenience assumed to be at a constant course. It is argued that only the GHA-Dec updating technique (GD-UT) will give a correct position solution, whereas the pre-electronic Intercept Method (IM)-based technique and its modern least-squares variant (LSQ) and the Altitude updating technique (A-UT), are proxy techniques that may yield significant deviations in position solution. GD-UT can be used in combination with a double-sight solution method for two sights and with LSQ for two or more sights. IM, LSQ and a geometric or algebraic double sight solution method applied to simultaneous double sights give identical results but the simultaneous sights case is generally an abstraction and strictly not applicable on a moving vessel.

This paper presents research and development work on a novel system with a view of developing a navigation system for the guidance of visually impaired pedestrians. The prime aim of the new developed system is to enhance the mobility of visually impaired pedestrians using remote guidance provided by sighted person. However, the system can be used in many other applications. The system implementation is based on the integration of the Global Positioning System (GPS), Geographical Information System (GIS) database and a remote vision facility over the third generation (3G) of mobile networks, providing a technological platform enabling visually impaired pedestrians to be guided remotely by a sighted guide.

This paper presents further research on the SNUGL pseudolite-based navigation system presented in this journal in 2003. This system has centimetre-level accuracy, but has an error source arising from right-hand circularly polarized (RHCP) transmissions, unlike outdoor Global Positioning System (GPS). The GPS satellites and pseudolites use RHCP signals, and the polarization affects carrier-phase measurements according to the Line-of-Sight (LOS) vectors from transmitters to receivers. The RHCP error is eliminated by a double differencing process in outdoor GPS, but the error remains in the pseudolite-based system because the LOS vectors from transmitters to a reference and user receivers are different for the close transmitter constellations. This paper shows the RHCP effect on the pseudolite-based navigation system through simulations and experiments. It then shows the RHCP-compensation method improves the measurement and position accuracy by over 10%.

There is a significant error in the equation referring to the rule of thumb for ‘line-of-sight’ analogue transmissions given in the paper by William Cairns, in the May 2005 issue (Cairns, 2005). This incorrect formula has previously been quoted in other sources, and possibly the author obtained it from one of these. In addition, the term ‘line-of-sight’ implies geometrical optics, whereas use of the term ‘radio horizon’ would be more appropriate since this takes account of the extension of radio wave transmission distance over the earth's surface caused by the normal water vapour content of the lower atmosphere over sea.

In this study, a method is presented to maintain real-time positioning at the decimetre-level accuracy during breaks in reception of the measurement corrections from multiple reference stations. The method is implemented at the rover by estimating prediction coefficients of the corrections during normal RTK positioning, and uses these coefficients to predict the corrections when reception of the corrections is temporarily lost. The paper focuses on one segment of this method, the on-the-fly prediction of orbital corrections. Frequently, only a few minutes of data representing short orbit ‘arcs’ are available to the user before losing radio transmission. Thus, it would be hard for the rover to predict the satellite positions using equations of motion. An alternative method is proposed. In this method, GPS orbital corrections are predicted as a time series and are added to the initial positions computed from the broadcast ephemeris to compute relatively accurate satellite positions. Different prediction approaches were investigated. Results show that the double exponential smoothing method and Winters' method can be successfully applied. The latter, however, has a better performance. The impact of the data length used for estimation of the prediction coefficients and the selection of seasonal lengths in Winters' method were investigated and some values were recommended. In general, the method can give orbital correction estimation accuracy of less than 5 cm after 15 minutes of prediction. This will result in a positioning accuracy better than 5 cm.

The geometry of the skeg in the Escort Tractor Voith tugs is the result of a series of intense investigations in the forms of the tug and its fins, oriented to get a significant improvement in the forces on the towing line when the indirect method is used in the escort towing. For that, and with the aim of getting the best behaviour of this fin, a variety of options have been investigated for years, evaluating its merits in terms of lift force and complexity to reach the present designs, which are adapted to the functions which the tug is destined to carry out. With the object of optimizing the lift force in the skeg when the indirect method is used in the escort towing, and after long investigations, the Voith Turbo Marine has incorporated a rotating cylinder at the leading edge to its design in escort towing for the first time at the beginning of 2005. The leading edge is the part over which the water flow first falls upon in normal escort operation conditions, calling this new development Voith Turbo Fin (VTF) to the system as a whole (skeg and rotating cylinder). This fin is analyzed in this article especially with regard to its basis, ways of operation and efficiency of the novel joining rotating cylinder.

K. H. Zevering has recently proposed a way to calculate the position of a moving observer, from the intersection of position circles, that differs from the methods hitherto accepted by mariners1. Zevering concludes, from discrepancies between his own results and that of other methods, that those others are in error. However, the discrepancies result from incorrect assumptions in his proposed method, which give rise to erroneous answers. My comments will be restricted to the simplest case of two observations only, and will assume that the Earth is, for our purposes, spherical.

The standards for CSTDMA Class B AIS will shortly be published by the International Electrotechnical Commission and equipment will become available during 2006. The perceived benefits that Class B brings to leisure craft users and its relatively low cost will make it attractive in the market place. A rapid take-up of Class B use can therefore be expected. This paper considers the impact that increased use of Class B will have on users of Class A AIS that are compulsorily fitted to larger vessels to meet the requirements of the International Maritime Organization Safety of Life at Sea convention. The CSTDMA Class B system has been designed to prevent overloading of the AIS VHF data link. This is briefly reviewed but there are a number of other aspects that need to be considered. These include: the increased garbling of Class B messages compared to those of Class A; the problems accruing from the low update rate of Class B information; the increase in display information that will need to be managed; and the possible increase in inappropriate manoeuvres of leisure craft caused by misplaced reliance on AIS. As a result of the investigation the paper highlights the fact that Class B users must not assume that their own presence, in the form of Class B transmissions, will be particularly visible on the bridge of many SOLAS vessels. This will continue to be the case for many years into the future, until such vessels are mandated to carry radar with AIS target overlay capability.

Edmund Hadnett writing in his article AIS at the Front Line in the May 2005 issue of the Journal, attributes to me the argument that ‘the automatic radar plotting aid (ARPA) should remain the principal means of assessing a risk of collision’. ARPA has never been the principal means of assessing a risk of collision and I have never argued that it has been and should remain so. My argument concerns the merits respectively of ARPA and AIS as navaids or a fusion of data from both.

In order to avoid a collision it has clearly been proved that for most seafarers there exists a set of possible situations known as head-on, crossing and overtaking, while ignoring in a general way the behaviour they should carry out when their vessel is in conditions of restricted visibility. It is obvious that the existence of two different manoeuvre methods in relation to the prevailing visibility confuses the navigator. But there is still more. With the arrival of AIS, the present wording of paragraph d) of Rule 19 leaves the seafarer in a state of total uncertainty when deciding the appropriate manoeuvre, so its amendment must be dealt with as a maximum priority. The following paper tries to find a solution to the dilemma, even though this does not imply a radical change in the general conception that a mariner may have of collision avoidance.