An improved microprocessor-based navigational aid system for use onboard a vessel is disclosed. The system includes a microprocessor having peripheral memory devices, sensors for measuring variable parameters and thumb switches for inserting known values for fixed parameters. The microprocessor computes from these parameters, data which enable the optimum navigation of a vessel. In determining these data, the computation includes such factors as leeway and current set and drift, which are determined electronically. The system includes components for receiving information regarding the vessel's present position from a satellite-based external information source. That information is relayed to the microprocessor which uses it to periodically correct the positional information stored in the peripheral memory device. The system further includes a sensor for measuring the heel angle of the vessel, a first display providing alpha and numeric readouts of the direction of movement of the vessel with respect to the earth, and a second display simultaneously providing alpha and numeric readouts of a selectable quantity.