Abstract: 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 (within the receiver or across a PPS measurement interface) to reside outside the security boundary. In addition, two methods are disclosed (passive residual method and active residual method) 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.