We have always had a strange behavior when following a route on our NN3D on autopilot (Simrad). If there is cross current setting the boat so that the COG is different than the heading, every couple of minutes the AP swings off course so that the magnetic heading matches the route heading, then slowly comes back on course so that the COG matches the route heading.
This happens every time the XTE reaches 0. For example, the boat is heading East with a 3kt current flowing North. To maintain a route, the magnetic heading is 20* South of the route bearing and COG. XTE will be 0 with the COG line matching the route line, the AP will then swing off course 20* so that the Heading line matches the route line, the XTE will get much larger (say, 500'), the AP will slowly correct back to course until the XTE is 0 and the whole thing repeats. This causes an "S" shaped track offset from the route line.
This does not happen when the autopilot is receiving its navigation input from a computer charting program - only the NN3D.
Do I have a setting switched on that I can change to prevent this? Does anyone know what is causing this? It could be something to do with the AP, but it only seems to happen with the NN3D as a data source.
Mark
This happens every time the XTE reaches 0. For example, the boat is heading East with a 3kt current flowing North. To maintain a route, the magnetic heading is 20* South of the route bearing and COG. XTE will be 0 with the COG line matching the route line, the AP will then swing off course 20* so that the Heading line matches the route line, the XTE will get much larger (say, 500'), the AP will slowly correct back to course until the XTE is 0 and the whole thing repeats. This causes an "S" shaped track offset from the route line.
This does not happen when the autopilot is receiving its navigation input from a computer charting program - only the NN3D.
Do I have a setting switched on that I can change to prevent this? Does anyone know what is causing this? It could be something to do with the AP, but it only seems to happen with the NN3D as a data source.
Mark