What is "opencpn"

OpenCPN (Open Chart Plotter Navigator) is a free software project to create a concise chart plotter and navigation software, for use underway or as a planning tool. OpenCPN is developed by a team of active sailors using real world conditions for program testing and refinement.

OpenCPN uses GPS input data to determine the ship's own position and data from an AIS receiver to plot the positions of ships in the neighborhood.