disclosed are methods for minimizing the security boundary of pps gps receivers . the primary disclosure is a lump sum sa correction method . this method can be used to limit the security boundary to the portion of the gps receiver that generates the lump sum sa correction , allowing all pps measurement processing to reside outside the security boundary . in addition , two methods are disclosed by which the data content of the pps measurement interface is entirely unclassified when keyed , thereby limiting the security boundary to the gps receiver itself .