IS-GPS-705J
01-AUG-2022
5
3.2 Interface Identification
The carriers of the L5 are typically modulated by two bit trains in phase quadrature. One is a composite bit train
generated by the modulo-2 addition of a pseudo-random noise (PRN) ranging code, a synchronization sequence (see
paragraph 3.3.3.1.2), and the downlink system data (referred to as L5 CNAV (civil navigation) data), and the second is
modulated with a PRN ranging code and synchronization sequence (see paragraph 3.3.2.3) that differ from those used
with the L5 CNAV data.
Two PRN ranging codes are transmitted on L5: the in-phase code (denoted as the I5-code); and the quadraphase code
(denoted as the Q5-code). Code-division-multiple-access techniques allow differentiating between the SVs even though
they may transmit at the same L5 frequency. The SVs shall transmit intentionally "incorrect" versions of the I5 and the
Q5-codes when needed to protect the users from receiving and utilizing anomalous CNAV signals. These two
"incorrect" codes are termed non-standard I5 (NSI5) and non-standard Q5 (NSQ5) codes.
The PRN ranging codes I5
i
(t) and Q5
i
(t) for SV ID number i are independent, but time synchronized, 1 millisecond in
length, with a chipping rate of 10.23 Mbps. For each code, the 1-millisecond sequences are the modulo-2 sum of two
sub-sequences referred to as XA and XB
i
; their lengths are 8,190 chips and 8,191 chips, respectively that restart to
generate the 10,230 chip code. The XB
i
sequence is selectively advanced, thereby allowing the basic code generation
technique to produce different code sequences of 1-millisecond in length. Of these, 32 pairs are currently designated
for use by SVs and 5 pairs are currently reserved. Assignment of these code phase segments by SV ID number (or
other use) is given in Table 3-I. SV ID and PRN numbers are identical to those for the L1 and L2 signals as specified in
IS-GPS-200.
The 74 codes (37 I5-codes and 37 Q5-codes) are a selected subset of over 4,000 possible codes that could be generated
using the selective advance. The remaining codes are available for future use of additional SVs and/or other L5 signal
applications such as Satellite-Based Augmentation System (SBAS) satellite signals. Of the remaining codes, Section
6.3.4 provides a selected subset of codes with assigned PRN numbers.
3.2.1.2 Non-Standard Codes
The NSI5 and NSQ5 codes, used to protect the user from tracking anomalous navigation signals are not for utilization
by the user and, therefore, are not defined in this document. The SVs shall also be capable of initiating and terminating
the broadcast of NSI5 and/or NSQ5 code(s) independently of each other, in response to Control Segment (CS)
command.
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