GLOBAL POSITIONING SYSTEMS DIRECTORATE SYSTEMS ENGINEERING & INTEGRATION INTERFACE SPECIFICATION

3.2.1 Ranging Codes. Three PRN ranging codes are transmitted: the precision (P) code which is the principal navigation ranging code; the Y-code, used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated; and the coarse/acquisition (C/A) code which is used for acquisition of the P (or Y) code (denoted as P(Y)) and as a civil ranging signal. Code-division-multiple-access techniques allow differentiating between the SVs even though they may transmit at the same frequencies. The SVs will transmit intentionally "incorrect"
versions of the C/A and the P(Y) codes where needed to protect the users from receiving and utilizing anomalous navigation signals. These two "incorrect" codes are termed non-standard C/A (NSC) and non-standard Y (NSY) codes.
For Block IIR-M, IIF, and subsequent blocks of SVs, two additional PRN ranging codes are transmitted. They are the L2 civil-moderate (L2 CM) code and the L2 civil-long (L2 CL) code.

The SVs will transmit intentionally "incorrect" versions of the L2 CM and L2 CL codes where needed to protect the users from receiving and utilizing anomalous navigation signals. These "incorrect" codes are termed non-standard L2 CM (NSCM) and non-standard L2 CL (NSCL) codes. The SVs shall also be capable of initiating and terminating the broadcast of NSCM and/or NSCL code(s) independently of each other, in response to CS command. Users shall only use non-dummy satellites as defined via current broadcast almanac. See Section
20.3.3.5.1.2 and/or Section 40.3.3.5.1.2 for the definition of information about dummy satellites in the almanac.