Si-tex SP38, Installation And Operation Manual, Page: 142 / 193

Sitex SP38 Installation and Operation Advanced Operations
Document PN 29010101 V1r0
- 142 -
Correction
If the vessel is not directly on track when in NAV mode, there are three ways it can arrive at
the waypoint:
Setting Description Comments
CTS Course-To-Steer
The autopilot picks out the Course-To-Steer
information from the Navigation System.
This results in a straight-line course to the
destination waypoint. Cross-track error,
while displayed, is not used by the autopilot.
XTE Cross-Track Error
The autopilot steers from its current
heading so as to minimize Cross-Track
Error. No Course-To-Steer information from
the Navigation System is used.
Both
Course-To-Steer
plus
Cross-Track Error
The autopilot steers a modified course
taken from the Navigation System’s
Course-To-Steer. The amount of
modification is proportional to the amount of
Cross-Track Error. This method is generally
best for normal usage.
Table 6 – NAV Mode Correction
XTE Sense - Cross-Track Error Sense
Cross track error (XTE) contains two pieces of information. First, it contains the amount of the
error. Secondly, it gives information about the ‘sense’ of the error (i.e. whether the vessel is to
the left or the right of the line between the beginning and ending waypoints).
The NMEA (National Marine Electronics Association) has specified how a cross track error
should be identified; a left indicator (L) in the data sentence means the vessel should steer to
the left and a right indicator (R) means the vessel should steer to the right.
However, some manufacturers of GPS, chart-plotters, and other navigation devices and
systems have used the left indicator to mean that the vessel is to the left of the line, so the
vessel should steer to the right, and the right indicator to mean that the vessel is to the right
of the line, so the vessel should steer to the left,
Because of this confusion, the SP38’s handling of XTE can be switched from “Norm” (normal)
to “Rev” (reverse). In “Norm”, the autopilot will respond normally to the sense of the cross-
track error. In “Rev”, the autopilot will respond in the opposite way in response to the same
information.
Max Correction
When activating pre-stored routes with a large amount of initial cross track error, the autopilot
will steer the boat at almost 90° to the track, in order to get back on track as fast as possible.
In certain circumstances, this can appear as though the vessel is moving away from the
desired waypoint. Setting Max Correction to something less than 90° can minimize this
phenomenon, yet will get you back on track less quickly.