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StarNavPilot ®

Navigation software.

Free nautical charts. With StarNavPilot you will never need to buy charts. You can scan your own charts, capture them with a digital camera or download the free charts available in our download center..


Autopilot and GPS guidance:

In the software StarNavPilot, even if the term of autopilot is maintained for a better understanding, it is actually a GPS guidance. A simple autopilot connected to the StarNavPilot interface transforms it into an efficient system for GPS guidance.

Difference between autopilot and GPS guidance or satellite guidance:

The automatic piloting consists simply in maintaining the boat to a constant orientation between the axis of the boat and the magnetic North. This device does not guarantee that the boat will move in a straight line or it will reach the intended position. A boat rarely moves on its axis. Even if the ship maintains a constant orientation relative to magnetic North, it undergoes drift. These deviations are due to current, wind, and the effect of corrections of helm which are not necessarily the same on both sides of the course (especially on a sailboat). The boat will not reach the destination point and this technique can not be used to automatically maintain the vessel in a security zone or to prevent danger.

The GPS guidance, also known as the satellite guidance system because GPS uses a constellation of satellites, is a device for driving the ship from the starting point of a route (or route segment) to a point of destination (or a point of change of destination) named waypoint while keeping the boat precisely on a straight line from the starting point to the destination point. This system does not use magnetic north as baseline data. Even if a compass is used to orient the boat, heading information to follow the route vary continuously to keep the boat on a straight line. It would be possible as well to use a gyroscope without any reference to magnetic north. A route is not necessarily a straight line between two points, but may be the result of a succession of several sections of route to follow a channel with an unknown number of waypoints and route segments. The passage from a segment of route to another and the necessary change of course will be managed automatically.

The necessary information for GPS guidance on a route segment are:
The geographical coordinates of the starting point of the route segment.
The geographic coordinates of the destination point of the route segment.
The maximum deviation (XTE) allowed on this route segment.
And some permanent parameters mainly related to the type of boat as: the correction factor and the maximum rate of rotation allowed (ROT).

The correction factor determines the correction required applied to the course to reach the destination point in function of the deviation from the axis of the route and the maximum track error allowed for the route segment. Normally this parameter remains constant.  for example, a value of 10 ° results that if the ground course from the starting point to the destination point of the section of route is 30 °, the maximum XTE allowed on both sides of axis is 0.2 miles (370 meters), the boat is at this instant at 0.1 mile on the right of the axis of the route, the rudder will be set to drive the boat on the course 25 ° to come closer of the axis of the route section. This value will decrease as the boat will join the line of the route or increase if it is not enough to bring the boat on its way. A maximum value of correction is provided. This value will be used in case of abnormal or accidental XTE and is about 3 X the correction at the maximum XTE allowed on this section of route. In such a configuration the route deviation alarms of StarNavPilot will have already triggered to warn the crew of a problem requiring an intervention.

The maximum allowable rate of rotation (ROT) is expressed in Degrees / Minute (° / min.) and determines the maximum number of degrees of rotation that the system can require  for a correction of route(rarely reached for a correction of route) or for a change of direction at a changing from a route segment to another. This is important for following a route with several segments of route with important changing of course. If in a section of the road the course is 120 ° then followed by another segment of route oriented to 160 °, the boat must turn of 40 ° to the right at the waypoint. The brutal 40 ° rotation of a speedboat can cause a capsize. On a sailboat there is a risk of uncontrolled tack. Management of the ROT softens bend so that it presents no danger.

The default value of ROT is 180 ° / minute (The boat will then last 1 minute to perform a turn back) which is a relatively low value and adapted to a small boat (pleasure) but too large for a ship trade (cargo) which instead will use a value of about 12 ° / minute or less for a very large ship (tanker). To adjust this value to your boat, measure the time it takes, to turn back at normal speed with manual helm (on a sailboat use engine) and calculate the ROT (180/Tmin .). Use a value slightly higher than the result obtained (+50%) as a precaution as it is recommended that the maneuvers with autopilot are slower than with manual helm.
If a autopilot system perform several successive managements of ROT (software + autopilot) the resulting value of the ROT will always be slower one. ROT management at various levels of the chain of control is not a problem but if you find that you have a too much slow ROT and its configuration in the software does not work, check if the autopilot does not itself perform a adjustment of ROT. Set it to a higher value in the autopilot and proceed to a final adjustment in StarNavPilot settings.

Another particular point of GPS guidance must be perfectly managed: it is the final approach of the destination point of a route section before proceeding to the next section of route. If we let the system try to reach the destination point precisely to its position, the system will attempt to pass the vertical of the GPS antenna of the boat exactly on the point of destination to the nearest centimeter (compared to the position provided at this point even if the GPS accuracy is only a few meters). This is of course not possible for a boat and can cause during the final approach, important changes of course as we approach of the waypoint. To overcome this drawback the system will stop to adjust the course when it is at a distance of the waypoint less than the authorized XTE for this section of route. From that moment the course is kept constant until the boat has passed the point of destination. For a short period this is a simple autopilot system.

This explains why in StarNavPilot a route segment is not only characterized by a starting point and a destination point but also a maximum allowed course deviation. A route segment is not a line but a surface on which the boat can move to join the opposite side of the starting side closest to the middle. The width of this surface (2x the maximum allowed XTE) is a critical parameter for the management of GPS guidance as it will be used to determine both the rate of correction of course, the management of the change of section of route and the alarms of the supervising of the route. This maximum route deviation allowed will be chosen depending on the type of navigation authorized by the configuration of the navigation area: sailing in open water or accurate tracking of a tortuous channel surrounded by dangers.

While following a route with StarNavPilot, listening to the speech is very useful because it announces by advance when approaching a waypoint and communicates the information to monitor the following section of route to allow the crew to anticipate the change of route and prepare itself for a possible setting of sails or other. This announcement can also check whether the route change does not contradict the rules of steering and sailing against other boats present in the area.

StarNavPilot configuration to use GPS guidance:
The advanced autopilot functions can be used only via the interface StarNavPilot. On the Data tab, access the configuration of the positioning engine. Under NMEA check GPABP Autopilot and under Dongle / interface check NMEA StarNavPilot output.

Information provided to the autopilot: StarNavPilot uses the standard NMEA GPAPB sentence with checksum verification. This sentence send the XTE data. Some autopilots use this data to adjust the course. To avoid any interaction with the management of course made by StarNavPilot, this data will always set to 0.0. If only the autopilot is connected to the NMEA output of the StarNavPilot interface, it is best to activate only the GPAPB sentence and disable all others.

In all cases an autopilot system must always remain under the supervision of the crew who must be able to instantly deactivate it and resume manual control of the helm.


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