The principles of operation of the Relative Navigation (RELNAV) function of the Joint Tactical Information Distribution System (JTIDS), which are based on its highly accurate time-of-arrival measurement capability and on a specially structured architecture, are described. The system configuration for implementing the RELNAV function in the JTIDS Hughes Improved Terminal (HIT) is presented. The integration options for interfacing the LN-31 Inertial Navigation Set with the HIT are discussed, and the selected interface is described. The RELNAV operational modes, control requirements and major software functions are discussed. A detailed description is given of the selected RELNAV algorithm and the associated timing and sequencing design. The filter state vector extrapolation and update formulation, the inertial processing and the source selection algorithms are presented. Some simulation results are presented.