A computer apparatus for post positioning with a selected precision. The apparatus includes a GNSS post processor to post process reference GNSS carrier phases from a reference system and rover GNSS carrier phases from a rover receiver to compute a secure position for the rover receiver not available to a user. The apparatus includes a random process generator to generate a sequence of offset vectors to dither the secure position according to a computed dither level to provide the selected precision for a user-available position for the rover receiver.