A method, apparatus, and system are disclosed for providing modified orbital assistance data to a mobile station to determine its location using global navigation satellite system (GNSS). The modified orbital assistance data may include predicted orbital information for the GNSS satellites combined with antenna phase center offset data for one or more GNSS satellites. The antenna phase center offset data may indicate an offset distance from the center of mass of the GNSS satellite to a position on an antenna of the respective GNSS satellite. The modified orbital assistance data may be in an earth-centered earth-fixed (ECEF) frame of reference and the antenna phase center offset data may be in a body-centered frame of reference.