The tactical air navigation (tacan) system is scheduled to be phased-out in the future. This means that, as a system, it must be replaced with another navigation system. The navstar global positioning system (gps) has been designated as its replacement. However, how gps will do that has not been specified, especially when it comes to shipboard applications. It is the purpose of this sbir program to develop a gps system specification for a shipboard tacan replacement to ensure that gps can be properly implemented as the eventual replacement. In order to make gps an adequate replacement for tacan, it must be used in a differential mode in conjunction with a data link, which may be a conventional communi- cation system, or one that is implemented at one or two of the gps frequencies. In the later case, the data link becomes a pseudolite system. In the differential mode, both the transmitting system on the ship and the user equipment in the aircraft navigate using the same gps space vehicles. Whether or not a conventional data link or a pseudolite system is used, the ship locations (and velocities) are broadcast to the user equipment, providing that knowledge and making it possible for the user equipment to navigate with respect to the ship.