IS-GPS-200J
22 May 2018
3
3 REQUIREMENTS
3.1 Interface Definition.
The interface between the GPS Space Segment (SS) and the GPS navigation User Segment (US) includes two RF
links, L1 and L2. Utilizing these links, the space vehicles (SVs) of the SS shall provide continuous earth coverage
signals that provide to the US the ranging codes and the system data needed to accomplish the GPS navigation
(NAV) mission. These signals shall be available to a suitably equipped user with RF visibility to an SV.
3.2 Interface Identification.
The carriers of L1 and L2 are typically modulated by one or more bit trains, each of which normally is a composite
generated by the modulo-2 addition of a pseudo-random noise (PRN) ranging code and the downlink system data
(referred to as NAV data).
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.
3.2.1.1 P-Code.
The PRN P-code for SV ID number i, for i = 1 to 37, is a ranging code, P
i
(t), of 7 days in length at a chipping rate
of 10.23 Mbps. The 7 day sequence is the modulo-2 sum of two sub-sequences referred to as X1 and X2
i
; their
lengths are 15,345,000 chips and 15,345,037 chips, respectively. The X2
i
sequence is an X2 sequence selectively
delayed by 1 to 37 chips thereby allowing the basic code generation technique to produce a set of 37 mutually
exclusive P-code sequences of 7 days in length. Assignment of these code phase segments by SV ID number is
given in Table 3-Ia. (NOTE: previous versions of this document reserved PRNs 33 through 37 for other uses.
Due to increased system capability, PRNs 33 through 37 are being redesignated to allow for use by SVs.)
An initial almanac collected from P(Y)-code in the upper PRNs must be obtained from PRNs 35, 36, or 38
through 63.