An ephemeris is valid for only four hours; an almanac is valid with little dilution of precision for up to two weeks. [7] The receiver uses the almanac to acquire a set of satellites based on stored time and location. As each satellite is acquired, its ephemeris is decoded so the satellite can be used for navigation. Besides redundancy and increased resistance to jamming, a critical benefit of having two frequencies transmitted from one satellite is the ability to measure directly, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as the Wide Area Augmentation System or WAAS). Advances in the technology used on both the GPS satellites and the GPS receivers has made ionospheric delay the largest remaining source of error in the signal. A receiver capable of performing this measurement can be significantly more accurate and is typically referred to as a dual frequency receiver. X 1 ( t ) = d ( t ) ⊕ d ( t − 2 ) ⊕ d ( t − 3 ) ⊕ d ( t − 5 ) ⊕ d ( t − 6 ) X 2 ( t ) = d ( t ) ⊕ d ( t − 1 ) ⊕ d ( t − 2 ) ⊕ d ( t − 3 ) ⊕ d ( t − 6 ) d ′ ( t ′ ) = { X 1 ( t ′ 2 ) if t ′ ≡ 0 ( mod 2 ) X 2 ( t ′ − 1 2 ) if t ′ ≡ 1 ( mod 2 ) {\displaystyle {\begin{aligned}X_{1}(t)&=d(t)\oplus d(t-2)\oplus d(t-3)\oplus d(t-5)\oplus d(t-6)\\X_{2}(t)&=d(t)\oplus d(t-1)\oplus d(t-2)\oplus d(t-3)\oplus d(t-6)\\d'(t')&={\begin{cases}X_{1}\left({\frac {t'}{2}}\right)&{\text{if }}t'\equiv 0{\pmod {2}}\\X_{2}\left({\frac {t'-1}{2}}\right)&{\text{if }}t'\equiv 1{\pmod {2}}\\\end{cases}}\end{aligned}}} 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. GPS signals are broadcast by Global Positioning System satellites to enable satellite navigation. Receivers on or near the Earth's surface can determine location, time, and velocity using this information. The GPS satellite constellation is operated by the 2nd Space Operations Squadron (2SOPS) of Space Delta 8, United States Space Force.

CNAV messages begin and end at start/end of GPS week plus an integer multiple of 12 seconds. [26] Specifically, the beginning of the first bit (with convolution encoding already applied) to contain information about a message matches the aforesaid synchronization. CNAV messages begin with an 8-bit preamble which is a fixed bit pattern and whose purpose is to enable the receiver to detect the beginning of a message. The P code is public, so to prevent unauthorized users from using or potentially interfering with it through spoofing, the P-code is XORed with W-code, a cryptographically generated sequence, to produce the Y-code. The Y-code is what the satellites have been transmitting since the anti-spoofing module was set to the "on" state. The encrypted signal is referred to as the P(Y)-code.For the ranging codes and navigation message to travel from the satellite to the receiver, they must be modulated onto a carrier wave. In the case of the original GPS design, two frequencies are utilized; one at 1575.42 MHz (10.23MHz × 154) called L1; and a second at 1227.60MHz (10.23MHz × 120), called L2. GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages. The navigation messages include ephemeris data, used in trilateration to calculate the position of each satellite in orbit, and information about the time and status of the entire satellite constellation, called the almanac.

L1C is a civilian-use signal, to be broadcast on the L1 frequency (1575.42MHz), which contains the C/A signal used by all current GPS users. The L1C signals will be broadcast from GPS III and later satellites, the first of which was launched in December 2018. [1] As of January2021 [update], L1C signals are not yet broadcast, and only four operational satellites are capable of broadcasting them. L1C is expected on 24 GPS satellites in the late 2020s. [1] L1C consists of a pilot (called L1C P) and a data (called L1C D) component. [35] These components use carriers with the same phase (within a margin of error of 100 milliradians), instead of carriers in quadrature as with L5. [36] The PRN codes are 10,230 chips long and transmitted at 1.023Mchip/s, thus repeating in 10ms. The pilot component is also modulated by an overlay code called L1C O (a secondary code that has a lower rate than the ranging code and is also predefined, like the ranging code). [35] Of the total L1C signal power, 25% is allocated to the data and 75% to the pilot. The modulation technique used is BOC(1,1) for the data signal and TMBOC for the pilot. The time multiplexed binary offset carrier (TMBOC) is BOC(1,1) for all except 4 of 33 cycles, when it switches to BOC(6,1). A and B are maximal length LFSRs. The modulo operations correspond to resets. Note that both are reset each millisecond (synchronized with C/A code epochs). In addition, the extra modulo operation in the description of A is due to the fact it is reset 1 cycle before its natural period (which is 8,191) so that the next repetition becomes offset by 1 cycle with respect to B [32] (otherwise, since both sequences would repeat, I5 and Q5 would repeat within any 1ms period as well, degrading correlation characteristics).

In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite. GPS encodes this information into the navigation message and modulates it onto both the C/A and P(Y) ranging codes at 50bit/s. The navigation message format described in this section is called LNAV data (for legacy navigation). 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). An interesting side effect of having each satellite transmit four separate signals is that the MNAV can potentially transmit four different data channels, offering increased data bandwidth.

An immediate effect of having two civilian frequencies being transmitted is the civilian receivers can now directly measure the ionospheric error in the same way as dual frequency P(Y)-code receivers. However, users utilizing the L2C signal alone, can expect 65% more position uncertainty due to ionospheric error than with the L1 signal alone. [28] Military (M-code) [ edit ]The L1C pilot and data ranging codes are based on a Legendre sequence with length 10 223 used to build an intermediate code (called a Weil code) which is expanded with a fixed 7-bit sequence to the required 10,230 bits. This 10,230-bit sequence is the ranging code and varies between PRN numbers and between the pilot and data components. The ranging codes are described by: [37] L1C i ( t ) = L1C ′ ( t mod 10 230 ) L1C i ′ ( t ′ ) = { W i ( t ′ ) if t ′ < p i ′ S ( t ′ − p i ′ ) if p i ′ ≤ t ′ < p i ′ + 7 W i ( t ′ − 7 ) if t ′ ≥ p i ′ + 7 S = ( 0 , 1 , 1 , 0 , 1 , 0 , 0 ) W i ( n ) = L ( n ) ⊕ L ( ( n + w i ) mod 10 223 ) L ( n ) = { 1 if n ≠ 0 and there is an integer m such that n ≡ m 2 ( mod 10 223 ) 0 otherwise {\displaystyle {\begin{aligned}{\text{L1C}}_{i}(t)&={\text{L1C}}'(t{\bmod {10\,230}})\\{\text{L1C}}'_{i}(t')&={\begin{cases}W_{i}(t')&{\text{ if }}t'

Satellites are uniquely identified by a serial number called space vehicle number (SVN) which does not change during its lifetime. In addition, all operating satellites are numbered with a space vehicle identifier (SV ID) and pseudorandom noise number (PRN number) which uniquely identifies the ranging codes that a satellite uses. There is a fixed one-to-one correspondence between SV identifiers and PRN numbers described in the interface specification. [4] Unlike SVNs, the SV ID/PRN number of a satellite may be changed (also changing the ranging codes it uses). At any point in time, any SV ID/PRN number is in use by at most a single satellite. A single SV ID/PRN number may have been used by several satellites at different points in time and a single satellite may have used different SV ID/PRN numbers at different points in time. The current SVNs and PRN numbers for the GPS constellation may be found at NAVCEN. A dataless acquisition aid is an additional signal, called a pilot carrier in some cases, broadcast alongside the data signal. This dataless signal is designed to be easier to acquire than the data encoded and, upon successful acquisition, can be used to acquire the data signal. This technique improves acquisition of the GPS signal and boosts power levels at the correlator. The C/A code is transmitted on the L1 frequency as a 1.023MHz signal using a bi-phase shift keying ( BPSK) modulation technique. The P(Y)-code is transmitted on both the L1 and L2 frequencies as a 10.23MHz signal using the same BPSK modulation, however the P(Y)-code carrier is in quadrature with the C/A carrier (meaning it is 90° out of phase). Whereas the C/A PRNs are unique for each satellite, each satellite transmits a different segment of a master P-code sequence approximately 2.35x10 14 chips long (235,000,000,000,000 chips). Each satellite repeatedly transmits its assigned segment of the master code, restarting every Sunday at 00:00:00 GPS time. (The GPS epoch was Sunday January 6, 1980 at 00:00:00 UTC, but GPS does not follow UTC leap seconds. So GPS time is ahead of UTC by an integer number of seconds.)The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users. A goal of 2013 was established with incentives offered to the contractors if they can complete it by 2011. Wider bandwidth provides a 10× processing gain, provides sharper autocorrelation (in absolute terms, not relative to chip time duration) and requires a higher sampling rate at the receiver. CM is modulated with the CNAV Navigation Message (see below), whereas CL does not contain any modulated data and is called a dataless sequence. The long, dataless sequence provides for approximately 24dB greater correlation (~250 times stronger) than L1 C/A-code. The L5 band provides additional robustness in the form of interference mitigation, the band being internationally protected, redundancy with existing bands, geostationary satellite augmentation, and ground-based augmentation. The added robustness of this band also benefits terrestrial applications. [30]