Navigational plotting system

An improved navigational plotting system having convenient initialization and the ability to compute and superimpose hyperbolic time difference lines on a chart includes an X-Y plotter for plotting the position and course of a vessel on a navigational chart or a plain piece of paper, dictated by the reception of hyperbolic navigation signals. Ambiguity-free conversion to latitude and longitude is done automatically without the necessity of manually entering the initial latitude and longitude of the vessel. Remote initialization is established by placing the pen of the plotter on a known point or position on the chart and entering the latitude and longitude of this known position or its hyperbolic navigation coordinates, with all subsequent plot locations being referenced to this point on the chart. A set of default pre-stored conditions are utilized in the initialization procedure in which preset scale size and hyperbolic line parameters are assumed, with convenient override sequences being provided. The plotter includes a high accuracy rapid hyperbolic line drawing system utilizing a specialized approximation sequence for those charts which are not provided with hyperbolic navigation TD lines. A base line extension zone where hyperbolic position ambiguity is difficult to resolve (danger zone) is indicated by the preventing of the drawing of hyperbolic lines in this zone, thereby to give the navigator adequate indication that hyperbolic navigation readings may be ambiguous. A system is provided for rapid counter setting for the counters which drive the pen and also indicate vessel location or destination. This system utilizes differing rates for clocking the counters depending on whether the vessel or destination is on-chart or off-chart. As a further feature, velocity filtering prevents wild pen movement as a result of system transients. Additional auxiliary readout functions such as destination, range, velocity, time of arrival, heading, etc. are also provided.