The modulation method is binary offset carrier, using a 10.23MHz subcarrier against the 5.115MHz code. This signal will have an overall bandwidth of approximately 24MHz, with significantly separated sideband lobes. The sidebands can be used to improve signal reception. Unlike the C/A code, L2C contains two distinct PRN code sequences to provide ranging information; the civil-moderate code (called CM), and the civil-long length code (called CL). The CM code is 10,230 chips long, repeating every 20 ms. The CL code is 767,250 chips long, repeating every 1,500 ms. Each signal is transmitted at 511,500 chips per second ( chip/s); however, they are multiplexed together to form a 1,023,000-chip/s signal. The GPS satellites (called space vehicles in the GPS interface specification documents) transmit simultaneously several ranging codes and navigation data using binary phase-shift keying (BPSK). Each frame contains (in subframe 1) the 10 least significant bits of the corresponding GPS week number. [15] Note that each frame is entirely within one GPS week because GPS frames do not cross GPS week boundaries. [16] Since rollover occurs every 1,024 GPS weeks (approximately every 19.6 years; 1,024 is 2 10), a receiver that computes current calendar dates needs to deduce the upper week number bits or obtain them from a different source. One possible method is for the receiver to save its current date in memory when shut down, and when powered on, assume that the newly decoded truncated week number corresponds to the period of 1,024 weeks that starts at the last saved date. This method correctly deduces the full week number if the receiver is never allowed to remain shut down (or without a time and position fix) for more than 1,024 weeks (~19.6 years).

C/A i is the code with PRN number i. A is the output of the first LFSR whose generator polynomial is x → x 10 + x 3 + 1, and initial state is 1111111111 2. B is the output of the second LFSR whose generator polynomial is x → x 10 + x 9 + x 8 + x 6 + x 3 + x 2 + 1 and initial state is also 1111111111 2. D i is a delay (by an integer number of periods) specific to each PRN number i; it is designated in the GPS interface specification. [4] ⊕ is exclusive or.General features [ edit ] A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time. Since the FEC encoded bit stream runs at 2 times the rate than the non FEC encoded bit as already described, then t = ⌊ t ′ 2 ⌋ {\displaystyle t=\left\lfloor {\tfrac {t'}{2}}\right\rfloor } . FEC encoding is performed independently of navigation message boundaries; [27] this follows from the above equations. The original GPS design contains two ranging codes: the coarse/acquisition (C/A) code, which is freely available to the public, and the restricted precision (P) code, usually reserved for military applications.

Having reached full operational capability on July 17, 1995 [20] the GPS system had completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to "modernize" the GPS system. Announcements from the Vice President and the White House in 1998 heralded the beginning of these changes and in 2000, the U.S. Congress reaffirmed the effort, referred to as GPS III. Pre-operational signal with message set "unhealthy" until sufficient monitoring capability established The delay for PRN numbers 34 and 37 is the same; therefore their C/A codes are identical and are not transmitted at the same time [5] (it may make one or both of those signals unusable due to mutual interference depending on the relative power levels received on each GPS receiver).Modernized GPS civilian signals have two general improvements over their legacy counterparts: a dataless acquisition aid and forward error correction (FEC) coding of the NAV message.