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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. Satellite data is updated typically every 24 hours, with up to 60 days data loaded in case there is a disruption in the ability to make updates regularly. Typically the updates contain new ephemerides, with new almanacs uploaded less frequently. The Control Segment guarantees that during normal operations a new almanac will be uploaded at least every 6 days. 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).
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}}}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.
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). 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. It uses forward error correction (FEC) provided by a rate 1/2 convolutional code, so while the navigation message is 25-bit/s, a 50-bit/s signal is transmitted. 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.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. 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). 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. 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. 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 ]
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). The arguments of the functions therein are the number of bits or chips since their epochs, starting at 0. The epoch of the LFSRs is the point at which they are at the initial state; and for the overall C/A codes it is the start of any UTC second plus any integer number of milliseconds. The output of LFSRs at negative arguments is defined consistent with the period which is 1,023 chips (this provision is necessary because B may have a negative argument using the above equation). 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). The interface to the User Segment ( GPS receivers) is described in the Interface Control Documents (ICD). The format of civilian signals is described in the Interface Specification (IS) which is a subset of the ICD.A major component of the modernization process is a new military signal. Called the Military code, or M-code, it was designed to further improve the anti-jamming and secure access of the military GPS signals. The P-code is a PRN sequence much longer than the C/A code: 6.187104x10 12 chips. Even though the P-code chip rate (10.23 Mchip/s) is ten times that of the C/A code, it repeats only once per week, eliminating range ambiguity. It was assumed that receivers could not directly acquire such a long and fast code so they would first "bootstrap" themselves with the C/A code to acquire the spacecraft ephemerides, produce an approximate time and position fix, and then acquire the P-code to refine the fix.
The second advancement is to use forward error correction (FEC) coding on the NAV message itself. Due to the relatively slow transmission rate of NAV data (usually 50 bits per second), small interruptions can have potentially large impacts. Therefore, FEC on the NAV message is a significant improvement in overall signal robustness. 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. 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.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. 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 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).
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