Another Port Phillip Crossing Attempt - Post COVID Lockdown
Checks done. Ready to go again. |
Steering to a Waypoint
When waypoints are only a few hundred metres away, it is simple to steer a course representing the Bearing to Waypoint (BTW) and you will typically arrive there, having followed closely along the rhumb line.
When the waypoint is many miles away, and the steering method is to steer the BTW, then it is quite likely that the vessel will deviate great distances from the rhumb line over a period of time.
This was one of the problems encountered during the last voyage across Port Phillip in March 2020.
The main software design change since then was to continual measure the Cross Track Error (CTE) and use this to add a proportional course correction to the BTW to encourage the vessel to return to the rhumb line. The amount of course correction to apply is calculated as the ratio of CTE to CTEMax (where CTEMax is defined in the mission as half width of the corridor to the next waypoint), multiplied by gain constant.
Hence the simple course to be steered is: BTW - CTE/CTEMax x Kc
Note: This is all assuming that a course can be laid directly to the waypoint without tacking. If tacking is required, then a whole set of different rules apply which are beyond the scope of this article.
The first 10 hours were good
Satellite Tracking Plot |
Conclusions
The new method of steering using CTE combined with BTW appears to work well.
I've tried to sail across Port Phillip twice now, and failed both times due to human intervention.
I'll need to add signage suggesting to "KEEP CLEAR" or head out to the open ocean where there's less chance of meeting other vessels.
Port Arlington - Home Port for the last vessel to "rescue" Voyager |
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