Abstract: The engineering design for broadcast ephemeris parameters fitting arithmetic and their interface are studied.Through the analysis of normal equation by condition number and parameters relativity,the small inclination is protected to be the primary reason that causes the inapplicability of GPS Kepler orbit elements ephemeris fitting arithmetic to fit GEO orbit.And more,the characteristic of earth-stationary orbit leads the value of parameter Crc or Crs out of the GPS ephemeris interface limits,so they must be enlarged twice for GEO.At last,a new arithmetic,named parameters restriction adjustment with dynamic weighting,is proposed for morbid normal equation problem of navigation ephemeris fitting.The experiments show that the new arithmetic can stabilize the fitting results,limit all the ephemeris results within the interface bounds as well.