GB2474715A

GB2474715A - Aiding navigation of a marine vessel in a tidal region

Info

Publication number: GB2474715A
Application number: GB0918779A
Authority: GB
Inventors: Brian Leslie Russell Stannah; Craig Rochford; Anthony Galley
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-10-26
Filing date: 2009-10-26
Publication date: 2011-04-27
Anticipated expiration: 2029-10-26
Also published as: GB201308411D0; GB2500325B; GB2500325A; GB2474715B; GB0918779D0

Abstract

An electronic navigation apparatus is provided for aiding navigation of a marine vessel comprising: a means for determining the position, speed and direction of the vessel; a means for calculating a predicted arrival time at points in a monitored zone, the zone being selected based on at least the position of the vessel; means for calculating the water depth at points in the monitored zone, by applying tide data to depth data for the respective predicted arrival times; and means for determining whether the expected water depth is sufficient for safe passage of the vessel. A GPS receiver may be used to determine the position, speed and direction of the vessel. An electronic navigation chart may provide depth information. The predicted arrival time may take into account tidal flow rates. If the expected depth is insufficient for safe passage of the vessel, an alert may be generated. Calculated expected depths may be calibrated via measured depths from a depth sounder. In a further embodiment a navigation arrangement is provided for determining whether the marine vessel is expected to be able to pass beneath an overhead obstruction.

Description

NAVIGATION DEVICE, NETWORKED APPARATUS, METHOD AND COMPUTER

PROGRAM FOR AIDING NAVIGATION OF A MARINE VESSEL

FIELD OF INVENTION

The present invention provides a navigation device, networked apparatus and a method and a computer program for aiding navigation of a marine vessel. In particular, the invention aids navigation in tidal regions and in regions for which the data underlying publicly available navigation chart data has limitations.

BACKGROUND

Navigating a marine vessel in shallow water requires great care to avoid accidental grounding, which can both damage and delay passage of the vessel. Accidental ground ings can put lives at risk as well as resulting in expensive damage, and can have adverse impacts on the marine environment.

In commercial shipping, the delay and damage resulting from grounding of a vessel can have huge commercial impacts -on the shipping company and their insurers and on the individuals and companies who are relying on timely delivery of the ship's contents. If a large commercial ship is badly damaged due to a grounding incident, there is a risk of major environmental damage as well as major costs to repair the ship and loss of revenue until the ship can return to service. There are also risks when helming a sailboat or navigating shallow waters in a motor-powered pleasure boat.

Depth charts are publicly available for coastal waters and navigable parts of many rivers, as well as some shallow international waters, and are in wide use by marine vessel helmsmen to help them plan their route through shallow waters. The combination of these charts and visible channel markers make a huge contribution to the avoidance of grounding incidents, but the publicly available navigation charts have major limitations. Some available charts are outdated (for example if sandbanks have moved or river banks have been eroded or silt has settled in shallow water since an area was last surveyed) and it is very common for the available charts to lack the level of detail that is required by small vessels when navigating shallow waters. For example, there may be insufficient data points to identify the shallowest points or to accurately show a navigable channel. Also, typical charts are based on the time of low tide for spring' tides (i.e. when there is a maximum impact from the moon's gravitational attraction) for a particular atmospheric pressure, and would provide a very conservative view of the available water depth if they could be relied on. However, the inadequate number of data points, the fact that some charts are out of date and atmospheric pressure variations combine to make it inadvisable to rely solely on currently available charts.

The effect of these various limitations is that each helmsman is required to make a great many judgements when planning a route and when helming a marine vessel. Sometimes these judgments will be made cautiously, leading to excessive journey times or excessive fuel use, or prompting a sailor or fisherman to return to port earlier than was necessary. On other occasions, an incorrect judgement will result in accidentally grounding and damaging the vessel, with the consequent risks mentioned above.

For these reasons, and since only specific depth points are shown on the charts, helmsmen tend to use a depth sounder (e.g. using sonar) to measure the current depth below a boat's keel, or the helmsman must interpolate between the points on a navigation chart that has specified depths, in order to estimate depths nearby.

Even with a depth sounder, each helmsman must make judgements regarding whether, when, and how to navigate his/her vessel through relatively shallow water, typically using his/her knowledge of the local topography, the available charts and tides, and information from a depth sounder. The helmsman must be willing to change the planned route when a grounding risk is identified from depth readings. A typical helmsman will monitor depth measurements, look out for visual hazards and watch other vessels and yet, because of the limitations of the available charts and the difficulty calculating current depths from tide tables, mistakes are often made.

For some helmsmen, the available data and charts are simply too difficult to use while also steering a vessel.

SUMMARY

A first aspect of the invention provides an electronic navigation apparatus for aiding marine vessel navigation in a tidal region, comprising: means for determining the marine vessel's position, speed and direction; means for calculating a predicted arrival time at points in a monitored zone, which zone is selected based on at least the position of the marine vessel; means for calculating an expected water depth at points in the monitored zone, by applying tide data to depth data for the respective predicted arrival times; and means for determining whether the expected water depth is sufficient for safe passage of the marine vessel.

In one embodiment, the calculation of predicted arrival times takes account of a current tidal flow rate and estimated variations in tidal flow rate over time to estimate the varying effect of tidal flow on the speed of the marine vessel. This estimation of tidal flow effects, the resulting calculations of predicted arrival times and expected depths at those times, and the determination of sufficiency of depth, can be repeated frequently during navigation of the marine vessel.

An electronic navigation apparatus according to a first embodiment of the invention can be used to predict the times during which a particular marine vessel can safely pass through shallow water, and to estimate the required speed and direction of the vessel to enable safe passage, and to determine whether an alternative route is required. In many cases, use of the electronic navigation apparatus according to the invention will remove reliance on conservative navigation charts that only show depths at lowest tide, such that journey times and fuel can be reduced, but navigation apparatus according to the invention can also help to avoid grounding the vessel.

The possibility of reduced times and costs can be very important for commercial vessels, and avoidance of grounding is very important for all vessels.

The apparatus according to a preferred embodiment includes means for generating an alert when an expected water depth is insufficient for safe passage of the marine vessel.

By looking ahead by a predefined time period or distance and limiting predictions of future depth and potential grounding alerts to a defined zone in the forward direction of the marine vessel, the recalculations of expected depths can be focussed on areas of current interest to device user, and can be repeated more quickly or with less processing power. The invention enables repeated determination of sufficiency of depth of water during navigation.

In one embodiment of the invention, a GPS receiver is provided within a navigation aid device, to determine a marine vessel's position, speed and direction of travel. The navigation aid device includes at least one processor and an associated system memory, and a display. A computer program is executed by the processor to select and then process a set of data points from an electronic navigation chart, starting from a selected first data point or points, in an order that optimizes the relevance of results (i.e. optimizes the relevance of the determination of sufficiency of depth, and the relevance of any generated alerts). This optimization can vary according to a device user's objective. For example, the objective may be to determine sufficiency of depth with particular emphasis on the area immediately in front of a marine vessel, and then looking forward in the direction of travel; and in this case the selected first data point or points are data points within the monitored zone closest to the vessel. Alternatively, the device user may wish to focus attention on an area that is a predefined time or distance ahead of the current time or position of the marine vessel, and to determine the sufficiency of depth at that point; and in this case the selected first point is a data point selected to achieve that objective. A device according to one embodiment of the invention provides the device user with configurable options for selection of the first data point according to how far ahead of the current position the device user wishes to focus his or her attention -with the user able to specify a screen location or a look-ahead distance or a look-ahead time. An advantage of providing such options is that calculations can be performed and results displayed relatively quickly for an area that the user currently wishes to focus attention on.

The nautical navigation aid device according to one embodiment of the invention executes a computer program implementing an expected-depth calculation algorithm, which processes input depth data from an electronic navigation chart and tide data, to provide a capability for repeated recalculation of expected depths at predicted arrival times, in at least the current forward direction of the marine vessel. This provides the device user with a real-time response to changes of vessel speed and/or direction. This may be shown on a display screen of the device by either displaying a new alert (for example, changing a green screen display representing a navigable zone to a red display indicating that the zone will not be navigable at the predicted time of arrival if the vessel maintains its new speed) or cancelling a displayed alert (for example, red zone changed to green zone).

As one example, to navigate an area of shallow water on an outgoing tide, it may be necessary to increase speed in order to pass the shallowest point before a cut-off time at which grounding could occur. The effect of an acceleration, that will allow the vessel to safely pass a shallow point when it otherwise could not, can be indicated to the device user by cancelling an alert (e.g. changing a red screen display to green).

Some of the embodiments described above use depth information from electronic navigation charts (ENCs), which data is widely available and widely used for nautical navigation, but a navigation apparatus according to one embodiment of the invention includes an input means enabling ENC data to be supplemented with other bathymetric data -for example from tidal gauges. Tidal gauges are known that publish data via the Internet or via VHF radio transmissions, but a navigation apparatus according to one embodiment of the present invention is adapted to receive various bathymetric data inputs (via satellite communications, GSM, WiMAX or other signalling solutions) and to calculate expected depths with reference to the input data.

In another embodiment, a navigation device according to the invention allows a user to input start times for a journey between specified points, and the device calculates a possible route and the required speed to complete the journey without grounding, taking account of the effect of tide on the water depth at the relevant times.

In one embodiment, the means for calculating an expected water depth processes a plurality of inputs, including measured water depth information, depth values or other bathymetric data from electronic navigation charts, and tide data, to provide improved prediction of future water depths. The measured water depth can be compared with stored bathymetric data from an electronic navigation chart and stored tide data, and used to calibrate or otherwise enhance the stored data. The enhancement of stored data may comprise storing additional data points in association with an electronic navigation chart for increased granularity (i.e. more detail of a specific area surveyed' by use of the present invention by the marine vessel). Secondly, the enhancement may comprise saving a corrected value in association with a stored data point.

Such corrections may be localized, such as if the chart fails to show a new wreck or if a sandbank has moved or a river bank has been eroded. Alternatively, corrections may be generally applicable to the area of the chart for the day on which the device is being used, if substantially all measurements show a consistent discrepancy between calculated and measured depths. This may apply if correcting stored data for the current atmospheric pressure, and/or if stored estimated depths were based on the "rule of twelfths" instead of accurate depth measurements at different states of tide. The enhanced data may be stored in a data storage device associated with the electronic navigation aid.

In one embodiment, a set of depth readings taken by a number of different vessels at different states of tide can be uploaded to a central computer that manages a navigation database. The different readings can be averaged for a particular state of tide to provide more reliable, updated depth measurements. The average taken from the measured data can then be distributed to multiple users, for example on a subscription basis. This averaging, for different states of tide, avoids a problem of too much emphasis being placed on unreliable individual soundings.

The means for determining the vessel's position, direction and speed may be a GPS system, or another positioning system receiving input signals from a plurality of satellites that have known positions and using the determined distance of the vessel from each of the satellites to calculate the vessel's position. The position, speed and direction information from the positioning system can be saved to a memory of the electronic navigation device, and can then be input to a processor within the device together with stored electronic navigation chart data and tide data The device's processor then executes a computer program that predicts water depths in the direction of travel of the vessel at the predicted arrival time of the vessel.

The generated alert may comprise a warning sound or a visual indication on a display screen, such as using colour coding to show safe and unsafe regions for the vessel within the direction of travel on a displayed navigation chart.

In one embodiment of the invention, tide data is integrated within an electronic navigation chart, providing more useful depth information than conventional charts showing depths at low tide.

In another embodiment, measured depth information is combined with depth information obtained from an electronic navigation chart that also includes tide data, and is then stored by the electronic navigation device. In this way, the device learns' about navigable safe routes and unsafe routes by storing modified navigation chart data and/or tide data for future use.

Another aspect of the invention provides a method for aiding marine vessel navigation, comprising the automated steps of: determining the marine vessel's position, speed and direction; calculating a predicted arrival time at points in a monitored zone, which zone is selected based on at least the position of the marine vessel; calculating an expected water depth at points in the monitored zone, by applying tide data to depth data for the respective predicted arrival times; and determining whether the expected water depth is sufficient for safe passage of the marine vessel.

In one embodiment, the method further comprises generating an alert when an expected water depth in a forward direction of the marine vessel is insufficient for safe passage of the marine vessel. In an alternative embodiment, the method further comprises determining an alternative route when an expected water depth in a forward direction of the marine vessel is insufficient for safe passage of the marine vessel.

One embodiment of the invention provides computer program code that is executable by a data processor to implement the steps of calculating a predicted arrival time, calculating an expected water depth and determining whether the expected depth is sufficient. The program code can be made available on a recording medium, or made available for transfer across a network communication path via a data transfer medium.

Another aspect of the invention provides an electronic navigation apparatus for aiding marine vessel navigation in a tidal region, for a marine vessel having a known maximum height above the surface of the water. The apparatus comprises: means for determining the marine vessel's latitude and longitude, direction and speed; means for calculating a predicted arrival time of the vessel at the latitude and longitude location of an overhead obstruction, wherein the overhead obstruction has a known latitude, longitude and obstruction minimum altitude; means for calculating an expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; means for calculating the expected distance between the water surface and the obstruction minimum altitude, based on the expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; and means for comparing the calculated expected distance with the maximum height of the marine vessel above the surface of the water, to determine whether the marine vessel is expected to be able to pass beneath the overhead obstruction.

Another aspect of the invention provides a method for automated determination of whether a marine vessel, having a known maximum height above the water surface, is expected to be able to pass beneath an overhead obstruction at a future time, comprising: determining the marine vessel's latitude and longitude, direction and speed; calculating a predicted arrival time of the vessel at the latitude and longitude location of an overhead obstruction, wherein the overhead obstruction has a known latitude, longitude and obstruction minimum altitude; calculating an expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; calculating the expected distance between the water surface and the obstruction minimum altitude, based on the expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; and comparing the calculated expected distance with the maximum height of the marine vessel above the surface of the water, to determine whether the calculated expected distance is greater than the maximum height.

BRIEF DESCRIPTION OF DRAWINGS

One or more embodiments of the invention are described below in more detail, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of apparatus for aiding marine navigation according to an embodiment of the invention; Figure 2 is a simplified representation of an example screen display on a display screen of an electronic navigation device according to an embodiment of the invention; Figure 3 shows a recalculated route in response to alert conditions; Figure 4 shows the steps of a method for calculating an expected depth at a future time, according to an embodiment of the invention; Figure 5 represents the effect of tide on the direction of travel of a marine vessel and on how far the vessel is expected to reach within specific time periods; and Figure 6 shows an alternative method for determining the adequacy of depths at future times, which combines measured depth data with calculated expected depths at future times to obtain a corrected or calibrated expected future depth.

DESCRIPTION OF ONE OR MORE EMBODIMENTS

Components of a nautical navigation aid device 10 according to one embodiment of the invention are shown schematically in Figure 1. The device includes a GPS receiver 20 comprising an antenna, for receiving signals from a plurality of satellites, and means for passing data representing distances from respective satellites to a processor 30 to calculate the device's position from these distances. The Global Positioning System (GPS) is a satellite-based global navigation system that is widely used to provide positioning information anywhere on the Earth.

The GPS uses several (e.g. 24 or 32) satellites orbiting the Earth, which transmit time-stamped signals for receipt by a very large number of GPS receiver devices, and uses a plurality of control and monitoring stations. The satellites broadcast signals that are used by the GPS receivers to provide three-dimensional location information (latitude, longitude, altitude) and accurate time. A GPS receiver calculates its position by timing the transmission delay of signals from the GPS satellites to calculate a distance to each of the satellites. A triangulation technique uses the calculated distances and knowledge of three or more satellites' positions to determine the position of the receiver (and to correct for minor GPS receiver clock errors using a fourth satellite's signal). Changes in position allow speed and direction of travel to be determined by a GPS receiver. GPS is a well known technology for use in positioning and navigation in aviation, land and marine applications, and GPS receivers are well known in the art, so the GPS receiver will not be described in more detail herein.

Other satellite-based positioning solutions are also known and could be used as an alternative to GPS, but this description will refer to a GPS receiver by way of example and for ease of reference.

The GPS receiver 20 and processor 30 are powered by a power source 40, which also provides power to other components including a clock 45. A display screen 60 is provided to display navigation charts and the marine vessel's position (i.e. the navigation device's position) within the displayed chart area, as well as to provide a visual indication of allowed safe routes and zones as well as zones of the chart over which safe passage of the vessel appears to be impossible (at the expected arrival time of the vessel as calculated from the current position, direction and speed of the vessel). User inputs to the navigation device 10 are via a user input device 50, which in the present invention comprises a set of keys for data entry and for selecting items from a menu of optional operations performed by the device. These selectable items can include functions for controlling what is displayed on the display screen 60, such as zoom and track right and left to provide a desired display area. The processor makes use of random access memory 70 during calculations, and electronic navigation charts 80 and tide data 90 and a future depth prediction algorithm 100 are held in non-volatile data storage 110 and are retrieved into RAM 70 when required for processing by the processor 30.

Also shown in Figure 1 is a depth sounder 120 that uses sonar to measure water depth. In this embodiment, the depth sounder 120 is provided on a marine vessel 200 as a separate component from the nautical navigation aid device 10, but is connected to the navigation aid device 10 (by either wired or wireless connections) so as to provide input data to the navigation aid device. As is well known in the art, sonar is used in nautical navigation for water depth measurements and to measure an underwater surface topography. Sound signals are transmitted, and reflected echo' signals are monitored, to determine distances to objects such as rocks and the sea bed. Echo sounders were first proposed for underwater use almost 100 years ago and are widely used in marine navigation, so the depth sounder (sonar transmitter and receiver and distance calculation) will not be described in detail herein.

The navigation aid device 10 may be a hand-held device or may be bolted to the marine vessel, but in either case is most conveniently provided as a compact device containing all of the processor, GPS receiver, display screen, input device, random access memory and non-volatile storage within a durable and waterproof casing. In other embodiments of the invention, the components shown schematically in Figure 1 may be provided as components of a network-connected on-board navigation system that integrates various navigation features that are additional to the present invention. For example, one such navigation system integrates sonar and radar and includes mechanisms for on-demand retrieval of chart data from a remote server computer when a marine vessel passes into a charted geographical area that is outside the area of the currently loaded chart. The present invention is not limited to one particular device type, and the term electronic navigation apparatus is intended to include components of an integrated navigation system as well as hand-held and other compact devices.

In Figure 1, a single processor 30 is shown, and this may execute a position determining algorithm in response to signals received by the antenna as well as calculating expected depths, but it will be appreciated by persons skilled in the art that the GPS receiver 20 may include its own dedicated processor and a dedicated memory storing the position determining algorithm, instead of relying on the same processor as is used for expected depth calculations.

A single power source may be used by all components of the navigation device 10 that require electrical power, or more than one power source may be used. Electrical power connections are not shown in the figures.

The navigation device 10 has the capability to calculate a predicted arrival time of a marine vessel at points in the forward direction of the marine vessel, based on the position, speed and direction of the marine vessel as determined by the processor 30 using data from the GPS receiver 20. The device 10 is also capable of calculating an expected water depth at points in the forward direction of the marine vessel at the respective predicted arrival time at those points.

This involves referring to tide data and calculating the effects on tidal depth changes with reference to the depth information within an electronic navigation chart, for particular points at the respective predicted arrival time at those points. The device 10 then determines, from the draught of the vessel and the expected water depth at a predicted arrival time, whether the expected depth is sufficient for safe passage of themarine vessel. The device can then provide information to the device user regarding whether the marine vessel can pass safely in a particular direction, for example via on-screen alerts or sounding an alarm (e.g. one of a set of audible alert signals). In one embodiment, calculated expected depths can be calibrated by referring to depth measurements (using data input from the depth sounder 120) to improve accuracy of the calculated expected depths.

Figure 2 is a simplified representation of an example screen display 200 that can be displayed to a user of the navigation device. The screen display includes a representation of the position of the marine vessel 210 and contour lines representing particular depths of water on a navigation chart. Depth monitoring is carried out for a zone 230 of the chart in the direction of travel of the marine vessel. In this embodiment, the zone 230 is bounded by a pair of lines 240 that each form a predefined acute angIe 250 to the direction of travel 260 of the marine vessel.

When the calculated expected water depth at a predicted time of arrival anywhere in the zone 230 is determined to be insufficient for safe passage of the marine vessel, the unsafe area is indicated on the display screen by means of a colour-coded display. In Figure 2, a first area 270 shown hatched in the figure represents a shallow area over which it has been determined that the vessel will be unable to safely pass. A second shallower area 280 may be shown differently as represented in Figure 2 (for example, the device could use red, amber and green coloured areas), but the present embodiment only differentiates between two types of area -areas for which the determination of whether the marine vessel can safely pass was positive, and areas for which the determination was negative.

In the example of Figure 2, an angle of approximately 35 degrees from the direction of travel has been chosen, but smaller or larger angles are equally possible. The choice of the size of the zone 230 is based on the desire to provide the user with the most valuable information, which means balancing the completeness of the information presented with the speed of its calculation, and the choice of monitored zone may be selectable by the device user. The bounding lines 240 of the zone 230 can be explicitly represented on the display screen, but this is not essential.

In an alternative embodiment, the monitored zone may be an area centred on a selected point in the forward direction of the marine vessel that the vessel is predicted to reach at a predefined future time. For example, this selected point may be at a distance along the direction of travel that the vessel is expected to cover in 5 mihutes, for example, for a small vessel that is navigating shallow coastal water. For a large ship that has a much larger turning circle and which is travelling in less shallow water, the selected point may be at a distance from the vessel's current position that the vessel is expected to travel in 1 hour. In one embodiment, the selected centre point of the monitored zone and the size of the monitored zone are configurable by the device user, so that the user is able to specify the desired information about future depths. This allows the user to focus attention on a local area very close to the marine vessel when necessary, and to switch to considering parts of the journey that are further ahead when required.

Starting from the selected point in the forward direction, a first algorithm for determining sufficiency of depth iteratively compares data points from the electronic navigation chart with tide data for the predicted arrival time of the vessel, starting with the closest data point within the chart to the selected central point and then checking each of the nearest neighbour data points, and then the next nearest points. If we assume a square grid array of known data points, the algorithm would start with a first selected central point, and then consider the 8 nearest neighbour data points surrounding the selected central point, and then consider the next closest 16 points, and so on until the boundary of the monitored zone is reached (or until a predetermined time has elapsed since the last calculation was commenced, such that a new calculation should commence starting at the selected central point). A second algorithm identifies regions of an electronic navigation chart that can be reached by the marine vessel within specified times (e.g. 2, 4, 6 minutes as shown in figure 5) and assumes a common arrival time for all points within a region. An expected water depth is calculated for all points in the region for which navigation data is available, for the predicted arrival time for that region, to determine sufficiency of water depth.

Figure 3 shows the result of a method of calculating a safe passage route when the method of determining sufficiency of depth for safe passage in the current direction produces a negative result and alert conditions are identified. In the embodiment represented in figure 3, an alternative route 300 is identified which bypasses all areas of insufficient depth such as area 310 to reach a desired location 320. In Figure 3, the monitored zone is defined by a 90 degree angle on each side of the current direction of travel such that another area 330 of insufficient depth has been identified. Such a wide angle is non-optimal when looking a long time or distance ahead in the direction of travel as a larger monitored zone requires more time to calculate expected depths for the monitored zone. = A method for determining sufficiency of depth, and displaying the determined result, is shown in Figure 4. In the method according to this embodiment, a GPS device is used to determine 400 the position (latitude and longitude) of the marine vessel from received satellite signals.

Changes of the vessel's position derived from GPS measurements can also determine the vessel's direction of movement and speed relative to the Earth. This position information is then compared with electronic navigation chart data to determine 410 the position of the marine vessel on a navigation chart. In the present embodiment, the nautical navigation device has a set of one of more stored navigation charts held in non-volatile storage, together with tide data for the area covered by the chart, but in alternative embodiments any required navigation charts can be downloaded as and when required from a remote server computer system that has access to a repository of electronic navigation charts. In other navigation systems according to alternative embodiments of the invention, required charts can be loaded into the system by inserting a cassette or CD-ROM that holds the required charts.

In the embodiment of Figure 4, a monitored zone is defined by the speed and direction of travel of the marine vessel, the monitored zone comprising a zone in the forward direction of the marine vessel defined by a specified angle to the direction of travel and defined by a distance that the vessel is expected to cover in a defined period of time. A first data point or set of points within this monitored zone is identified 420 by looking ahead by a specified time, calculating the distance that the vessel is expected to cover in the direction of travel in the specified time, and identifying the nearest data point or set of points within the navigation chart's monitored zone.

The specified look-ahead time is zero if starting from the current position, and in one embodiment of the invention calculations always begin with data points in the monitored zone that are closest to the current location.

For the identified data point or set of data points, a predicted arrival time is determined 430, and then tide data relating to this predicted arrival time is retrieved 440 from the device's non-volatile storage. The tide data for the relevant time is applied 450 to the depth as indicated on the electronic navigation chart, to calculate the water depth at that point and time. The calculated depth is then compared 460 with the water depth that is known to be required for the particular marine vessel to achieve safe passage, to determine a positive or negative result as to whether the vessel can safely pass over the data point. Each determination of sufficiency or insufficiency of water depth is then displayed 470 on the navigation device's display screen byzrepresenting sufficient depth areas in green and insufficient depth areas in red.

Having determined a result for a first data point or set of points, and generated a suitable indication of expected sufficiency or insufficiency of depth at the predicted arrival time (e.g. red or green display), a next data point is selected 480 from the set of nearest neighbour data points in the electronic navigation chart or a new set of points is selected for a neighbouring region, and the calculation 430,440,450,460 and display 470 is repeated for the selected data point or points. This is repeated for other data points, extending outwards from the selected first data point or points, until either a timer expires 490 or all data points in a defined monitored zone have been processed. The process then starts from the beginning, checking the vessel position, speed and direction 400.

As mentioned above, in one implementation of the method shown in Figure 4, the first selected data point is the data point that is the closest point, of all data points within the monitored zone, to the determined position of the marine vessel. This selection of a close starting point ensures that the data that is calculated most quickly is the data that relates to water depths close to the marine vessel. This area will often be the area for which greatest accuracy and a highest frequency of displayed updates are required by device users.

In one embodiment of the invention, the selection of a first data point or a first set of points is configurable by device users according to how far ahead of the vessel's current position the user wishes to focus his or her attention. This may vary at different times -perhaps looking a very long distance and time ahead while planning a route, looking a short time period ahead while helming a marine vessel at high speeds in relatively deep waters, and focussing most attention only a very short distance ahead while navigating the vessel at low speeds in shallow water.

As shown in Figure 5, a vector 520 representing the speed and direction of travel of the vessel differs from the direction 500 in which the vessel is pointing (i.e. the longitudinal axis of symmetry passing through the vessel's bow). This difference is partly a result of tidal flow 510 under the vessel. Secondly, for a sailing boat, the direction of travel will be influenced by a sideways component of the forces generated by the sails, even if there is no tide flowing.

In Figure 5, the iector 520 represents the direction of travel and the distance that the vessel is expected to cover in 3 minutes. A set of boundary lines 530 represent how far the vessel is expected to be able to reach in 2, 4 and 6 minutes respectively, and such boundaries can be used to define different regions within a monitored zone of the electronic navigation chart. As shown, the tidal flow 510 results in the vessel being expected to reach further in the direction of the tidal flow than if the vessel travels against the tidal flow. Each pair of consecutive boundary lines 530 defines a region for which the vessel's arrival time can be estimated by a single time value, and the part of each of these regions that is within the monitored zone between the pair of boundary lines 540 (at a predefined angle from the direction of travel) is the part for which the expected depth is calculated. In other embodiments, where sufficient processing power is available to avoid the need to reduce computations, all directions of possible travel are monitored (i.e. the boundary lines 540 are not used); and the simplifying assumption of a fixed time value for each region within a monitored zone is also not used, but is replaced by an estimated arrival time for each point in the monitored zone.

In one embodiment, a set of data points within the monitored zone of an electronic navigation chart is identified and then a surface topography for the sea bed or river bed is generated by interpolation between the available depth information of the set of data points. A predicted arrival time is calculated for each region (bounded by consecutive lines 530 and boundary lines 540) and tide data for the predicted arrival time for each region is applied to the surface topography to calculate expected depths across the region.

Although a single arrow 510 represents tidal flow in Figure 5, it will be appreciated by skilled persons that tidal flow varies considerably for different points within a typical tidal estuary or river, as well as varying with time. Since tidal flow charts are available which show the variation in flow speeds with time and position for many tidal regions, these tidal flows can be included in the prediction of the vessel's arrival time at points for which the expected depths are calculated.

In one embodiment of the present invention, a tidal flow model for a known tidal area is used to determine the effect of changing tidal flows on the speed of the marine vessel at the predicted arrival time at each position along a route followed by the marine vessel. In a first approximation, a vector representing the changing surface speed of the tidal flow within the main navigable channel of a river is calculated and its impact on the speed of the vessel over the grouhd is calculated from this tidal flow and the vessel's currnt speed through the water. In this first approximation, rotary tidal flows are disregarded and our tidal flow model assumes a fixed direction of tidal flow within the main channel, with either a positive, zero or negative flow rates at any point in time.

The resultant predicted arrival time at shallow points in the tidal region is calculated 430, taking account of the tidal flow on the speed of the marine vessel. If the speed of the vessel through the water changes, the result may be a very different predicted arrival time. Even without changing speed through the water, each successive calculation of predicted arrival times will provide a refinement of the earlier calculations, as the vessel progresses along its route and hence a refinement of the calculated expected depths at the predicted arrival times at various points along the route. A rate of change of tide height is calculated from the current tidal height and known tidal height variation data for particular positions. Tidal height variations for a position of interest can be estimated by interpolating between known depths of known positions, and the estimated rate of change of height is then used in the calculation of expected depths 450. As with predicted arrival times, each successive recalculation of expected depths in the forward direction of the marine vessel is a refinement of the previous calculations.

Figure 6 shows an alternative method according to the embodiment, in which the step of accessing 440 tide data is performed in parallel with inputting 445 actual depth measurements from a depth sounder device 120. An additional comparison step comprises comparing 455 measured depths with depths as calculated in step 450, and correcting or calibrating 457 future depth predictions by applying the difference between the calculated expected depths and measured depths. That is, the calculated expected depths at future times are not limited to application of known tide data to known navigation charts, but also take account of measured depths and in particular the difference between calculated expected depths and measured depths.

The inventors of the present invention have identified limitations in publicly available navigation charts and tide data, and a number of causes of those errors. Firstly, depths as surveyed on a particular day are dependent on the atmospheric pressure in the surveyed area on that day, and so on a different day the water depth may vary. If a number of depth measurements are taken for an area of interest, the difference between calculated expected depths and measured depths can be used to recalibrate stored depth values for the area to provide a more accurate determination of whether the marine vessel can safely pass over a shallow area.

Secondly, inaccuracies arise when the navigation charts being relied on are out of date. Coastal and river erosion and shifting sands can increase or decrease the water depth in different areas.

For an area of interest to the navigation device user, it can be desirable to save measured depth information for particular data points that are identified as being inaccurate in the electronic navigation chart that is being used. This can be saved as a correction value for each data point that is identified as inaccurate, such that the chart is displayed to the device user with a more accurate indication of depth and a more accurate determination of whether the vessel can safely pass. In one embodiment described above, an underwater surface topography is estimated by interpolating between available depth data and this is used in the assessment of expected depths. A forward-looking sonar device can be added to the apparatus described above to provide another input to the navigation aid, and can be used to measure the surface topography in front of the marine vessel. Sonar measurements can also be taken to the side of the vessel if required, enabling surveying of the sea bed in all directions for an area around the vessel. This measured surface topography and measured depth data can then be saved in a data repository in association with the electronic chart data so that more accurate depth and topography data is captured for areas that are of interest to the device user. That is, the user may choose to store bathymetric information for a shallow river mouth or tidal estuary that the user expects to have to navigate again in the near future, or to store data for which the measured data is very different from the information that is publicly available from navigation charts. The user may choose to discard measured data for deeper waters or waters for which the estimated topography and calculated expected depths are consistent with publicly available charts.

The present embodiment does not, however, respond to discrepancies between data from charts and measured data by accepting the measured depth data and disregarding the charts.

Instead, an offset is calculated from the differences between measured and predicted depths and the data provided by an echo sounder is then correlated with stored bathymetric data to enable more accurate calculations of expected depths. A depth offset correction factor can be calculated as between measurements from a depth sounder and stored depth data within an electronic navigation chart, filtering raw echo sounder data using a low pass or Kalrnan filter to reduce high frequency noise (resulting from ambient noise or multiple reflections from fish, etc) and comparing the filtered echo sounder data with predicted depth data. The depth offset can then be applied to future expected depth calculations. As is known in the art, depth sounders are themselves susceptible to a number of variations and inaccuracies, such as resulting from sediment at the sea or river bed, varying speed of sound waves according to water temperature and salinity, swell, pitch and roll of the vessel in waves, heeling of the vessel (especially for sailing yachts), or simply a poorly calibrated instrument. Survey vessels address these problems with a variety of techniques (swell compensators, TDS probes, multi-frequency transducers, stable hull designs, and so on). Any comparisons between measured depths and depth data from a chart is also susceptible to any errors in the determined position.

In view of the various factors set out above, the present embodiment applies correlation and calibration techniques for improved accuracy, rather than discarding either one of the measured data and published chart data.

The above-described embodiments involve a nautical navigation apparatus receiving various inputs, including satellite signals or other signals that can be used for determining position, speed and direction, and including measured depths from a depth sounder or another sea bed scanning apparatus on the vessel, or from nearby tidal depth gauges (e.g. transmitting via VHF). In certain embodiments tidal flow data is also input to the navigation apparatus.

Calculations are then made with reference to data from electronic navigation charts and the various inputs. In one implementation of the electronic navigation apparatus of the invention, the calculation of expected water depths is carried out on a data processing apparatus that is separate from a user's hand-held or on-deck navigation aid device. This separate data processing apparatus can be a computer on the marine vessel, which computer includes wireless or wired communication capability for sending the results of the determination of sufficiency of depth to hand-held and on-deck devices for display to the user. Alternatively, the separate data processing apparatus could be a server computer or network of server computers located remote from the marine vessel. Data transmission rates of many megabits per second are now available for transmission to and from mobile devices, whereas high power and low cost processors and memory are available for inclusion within a mobile device, and so it is possible to implement the present invention such that the data processing features are included on each hand-held device, or on a vessel's main computer, or more remotely.

Having described embodiments of the invention which are part of an integrated navigation system including radar, sonar and mechanisms for obtaining updated bathymetric data, it should be noted that the present invention can be integrated within an Electronic Chart Display and Information System (ECDIS) or with other passage planning and navigation systems. Current ECDIS implementations display information from electronic navigation charts (ENCs) and use positioning information from the Global Positioning system (GPS), but data from other navigational sensors such as radar and the Automatic Identification System (AIS) can also be integrated in a computer-based ECDIS navigation system that includes an implementation of the present invention.

Another application of the invention, which calculates expected water levels at predicted arrival times, is to determine whether the tide-effected water surface height is low enough for a vessel's highest point to pass below a bridge or other fixed structure. Let us assume that a particular vessel has a known maximum height above sea level, and that an obstruction's lowest point has a known altitude. For example, the obstruction minimum altitude may be the lowest point of a bridge under which marine vessels must pass (i.e. disregarding parts of the bridge under which vessels do not pass, such as parts outside of a marked navigable shipping channel). An electronic navigation apparatus and a method for aiding marine vessel navigation are provided for implementing the following steps: determining the marine vessel's latitude, S longitude, direction and speed; calculating a predicted arrival time of the vessel at the latitude and longitude location of an overhead obstruction, wherein the overhead obstruction has a known latitude, longitude and obstruction minimum altitude; calculating an expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; calculating the expected distance between the water surface and the obstruction minimum altitude, based on the expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; and comparing the calculated expected distance with the maximum height of the marine vessel above the surface of the water, to determine whether the marine vessel is expected to be able to safely pass beneath the overhead obstruction. The present invention solves the most challenging part of this is determination, by taking account of tidal flow to predict the arrival time of the vessel at the location of the obstruction and calculating the expected water depth at that time and location. It should be noted that the depth data that is available from typical navigation charts is for the lowest possible tide and so corresponds to the greatest possible distance between the water surface and bridges or other overhead obstructions. Therefore, minimum depth data of typical charts does not lead to a conservative assessment of whether a vessel can safely pass beneath an overhead obstruction, but would result in over-estimating the distance below a bridge. The highest tide water level could be used instead, but that provides only a minimum possible: distance between the water and the lowest point of the bridge or other obstruction. The present invention allows a determination to be made about whether there will be sufficient space under an obstruction at a future time, taking account of tides. A minimum safe height of tide and a maximum safe height of tide can both be calculated for relatively shallow water underneath an overhead obstruction, and the required time window to pass safely.

Claims

CLAIMS1. An electronic navigation apparatus for aiding marine vessel navigation in a tidal region, comprising: means for determining the marine vessel's position, speed and direction; means for calculating a predicted arrival time at points in a monitored zone, which zone is selected based on at least the position of the marine vessel; means for calculating an expected water depth at points in the monitored zone, by applying tide data to depth data for the respective predicted arrival times; and means for determining whether the expected water depth is sufficient for safe passage of the marine vessel.
2. An electronic navigation apparatus according to claim 1, wherein the means for calculating a predicted arrival time comprises data processing apparatus including a processor, a memory for holding tide and depth data to be processed, and an input means adapted to receive measurements of a current tidal flow rate and to transfer said measurements of a current tidal flow rate to the memory, the processor being controlled to execute a computer program to predict the effect of tidal flow on the speed of the marine vessel.
3. An electronic navigation apparatus according to claim 1 or claim 2, wherein the:meas for calculating a predicted arrival time comprises means for calculating an estimate of variations in tidal flow rate over time, and an estimate of the varying effect of tidal flow on the speed of the marine vessel over time, thereby to enhance the calculation of predicted arrival times at points in the monitored zone.
4. An electronic navigation apparatus according to claim 2 or claim 3, including a processor that is controlled to repeatedly execute a computer program, multiple times each minute, during navigation of the marine vessel in a tidal region, the computer program being adapted to calculate predicted arrival times and expected depths.
5. An electronic navigation apparatus according to claim 1 further comprising means for generating an alert when an expected water depth is insufficient for safe passage of the marine vessel.
6. An electronic navigation apparatus according to any one of the preceding claims, wherein the monitored zone is selected based on the marine vessel's position and direction, and comprises a zone within an electronic navigation chart that is in the forward direction of the marine vessel.
7. An electronic navigation apparatus according to any preceding claim, including means for identifying, when an expected water depth in a forward direction of the marine vessel is insufficient for safe passage of the marine vessel, an alternative direction to the current forward direction wherein the alternative direction is a direction for which the expected water depth is sufficient for safe passage of the marine vessel.
8. An electronic navigation apparatus according to any one of claims 1 to 6, including means for determining, when an expected water depth in a forward direction of the marine vessel is insufficient for safe passage of the marine vessel, a required speed for the marine vessel at which the expected water depth at the predicted arrival time will be sufficient for safe passage of the marine vessel.
9. An electronic navigation apparatus according to any one of claims 1 to 6, wherein the monitored zone comprises at least a partial planned route for the marine vessel.
10. An electronic navigation apparatus according to any one of the preceding claims, wherein the means for determining the marine vessel's position, speed and direction comprises a GPS receiver device for receiving signals from a plurality of satellites and for calculating the vessel's position, speed and direction from the received signals.
11. An electronic navigation apparatus according to any one of the preceding claims, further comprising: an input means for receiving data from a depth measurement device, which depth measurement device measures a depth in the vicinity of the current location of the marine vessel; and wherein the means for determining whether the expected water depth is sufficient comprises means for comparing at least one measured depth with at least one calculated expected depth, calibrating the calculated expected depths based on the difference between the at least one measured depth and the at least one calculated depth, and comparing the calibrated expected depths with a required depth for safe passage of the marine vessel.
12. An electronic navigation apparatus according to claim 11, including a depth sounder connected to the input means to provide measured depth data, calibrated to take account of the draught of the marine vessel, for comparison with at least one calculated expected depth.
13. An electronic navigation apparatus according to claim 11, wherein the input means includes a signal receiver for receiving depth measurements transmitted by the depth measurement device via a wireless data transfer mechanism.
14. An electronic navigation apparatus according to claim 11, wherein the depth measurement device includes a sonar device arranged to measure the surface topography of an underwater area; wherein the means for calculating an expected water depth combines the measured surface topography with a measured depth at the current location of the marine vessel to calculate depths for said underwater area.
15. A method for aiding marine vessel navigation, comprising the automated steps of: determining the marine vessel's position, speed and direction; calulating a predicted arrival time at points in a monitored zone, which zone is selected based on at least the position of the marine vessel; calculating an expected water depth at points in the monitored zone, by applying tide data to depth data for the respective predicted arrival times; and determining whether the expected water depth is sufficient for safe passage of the marine vessel.
16. The method of claim 15, further comprising: generating an alert when an expected water depth is determined to be insufficient for safe passage of the marine vessel.
17. The method of claim 15 or claim 16, further comprising: in response to determining that an expected water depth in a forward direction of the marine vessel is insufficient for safe passage of the marine vessel, identifying an alternative direction for which the expected water depth is sufficient for safe passage of the marine vessel.
18. The method of any one of claims 15 to 17, further comprising: selecting a subset of data points within an electronic navigation chart, the subset comprising data points that lie within a defined monitoring zone; wherein the step of calculating an expected water depth at points in the monitoring zone comprises interpolating between depth information of the selected data points of the monitoring zone to generate an approximate underwater surface topography for the monitoring zone, and calculating an expected depth for all points within the monitoring zone for the respective predicted arrival time.
19. The method of any one of claims 15 to 18, wherein the calculation of expected depths includes comparing at least one measured depth with at least one calculated expected depth, and calibrating calculated expected depths based on the difference between the at least one measured depth and the at least one calculated depth.
20. The method of claim 19, wherein the comparing comprises comparing a plurality of measured depths with calculated expected depths for respective points within a monitoring zone, evaluating whether the measured and calculated depths for respective points differ from eadh other by a substantially consistent difference value arid, if so, calibrating all calculated expected depths for the monitoring zone with reference to the difference value.
21. A computer program, comprising program code for controlling the performance of operations by a data processing apparatus to perform a method for aiding navigation comprising: determining the marine vessel's position, speed and direction; calculating a predicted arrival time at points in a monitored zone, which monitored zone is selected based on at least the position of the marine vessel; calculating an expected water depth at points in the monitored zone, by applying tide data to depth data for the respective predicted arrival times at said points; and determining whether the expected water depth is sufficient for safe passage of the marine vessel.
22. A computer program product comprising a computer program according to claim 21 recorded on a recording medium.
23. An electronic navigation apparatus for aiding marine vessel navigation in a tidal region, for a marine vessel having a known maximum height above the surface of the water, comprising: means for determining the marine vessel's latitude and longitude, direction and speed; means for calculating a predicted arrival time of the vessel at the latitude and longitude location of an overhead obstruction, wherein the overhead obstruction has a known latitude, longitude and obstruction minimum altitude; means for calculating an expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; means for calculating the expected distance between the water surface and the obstruction minimum altitude, based on the expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; and means for comparing the calculated expected distance with the maximum height of the marine vessel above the surface of the water, to determine whether the marine vessel is expected to be able to pass beneath the overhead obstruction.
24. A method for automated determination of whether a marine vessel, having a known maximum height above the water surface, is expected to be able to pass beneath an overhead obstruction at a future time, comprising: determining the marine vessel's latitude and longitude, direction and speed; calculating a predicted arrival time of the vessel at the latitude and longitude location of an overhead obstruction, wherein the overhead obstruction has a known latitude, longitude and obstruction minimum altitude; calculating an expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; calculating the expected distance between the water surface and the obstruction minimum altitude, based on the expected water depth at the latitude and longitude location of the overhead obstruction at the predicted arrival time; and comparing the calculated expected distance with the maximum height of the marine vessel above the surface of the water, to determine whether the calculated expected distance is greater than the maximum height.
25. The method of claim 24, including generating an alert when the calcu'ated expected distance is not greater than the maximum height.