This report presents a design concept for a high-precision system for harbor/harbor entrance navigation. The concept is based on differential operation of NAVSTAR GPS, whereby local corrections to the satellite signals are broadcast to mariners in a harbor area. The system is expected to provide continuous navigation service with a predictable (absolute) accuracy of 8-12 meters (2drms). The design is configured around a hypothetical system to cover New York Harbor. The report addresses the hardware configuration and costs of installing and maintaining the system. Also provided are guidelines for verifying the performance of the system.