GPS signals

Some limited use of restricted signals can still be made by civilians without decryption; this is called codeless and semi-codeless access, and this is officially supported.

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.

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.

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.

In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite.

The navigation message conveys information of three types: An ephemeris is valid for only four hours, while an almanac is valid–with little dilution of precision–for up to two weeks.

[14] Note that the 2 least significant bits can be safely omitted because one HOW occurs in the navigation message every 6 seconds, which is equal to the resolution of the truncated TOW count thereof.

Equivalently, the truncated TOW count is the time duration since the last GPS week start/end to the beginning of the next frame in units of 6 seconds.

In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process.

The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because as the receiver hardware becomes more capable, the time to lock onto the satellite signals shrinks; however, the ephemeris data requires 18 to 36 seconds before it is received, due to the low data transmission rate.

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 project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users.

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.

Due to the relatively slow transmission rate of NAV data (usually 50 bits per second), small interruptions can have potentially large impacts.

The current status of the L2C signal as of July 3, 2023[21] is: The civil-moderate and civil-long ranging codes are generated by a modular LFSR which is reset periodically to a predetermined initial state.

Only a small fraction of the available packet types have been defined; this enables the system to grow and incorporate advances without breaking compatibility.

[26] Specifically, the beginning of the first bit (with convolution encoding already applied) to contain information about a message matches the aforesaid synchronization.

The L5 signal provides a means of radionavigation secure and robust enough for life critical applications, such as aircraft precision approach guidance.

Both codes are 10,230 chips long, transmitted at 10.23 Mchip/s (1 ms repetition period), and are generated identically (differing only in initial states).

These codes are the combination (by exclusive-or) of the output of 2 differing linear-feedback shift registers (LFSRs) which are selectively reset.

For PRN numbers 1 to 63 they are the truncated outputs of maximal period LFSRs which vary in initial conditions and feedback polynomials.

Note that aliasing is sometimes intentional (specifically, when undersampling is used) but filtering is still required to discard frequencies not intended to be present in the digital representation.

Acquiring a signal is the process of determining the frequency and code phase (both relative to receiver time) when it was previously unknown.

The described procedure is based on computing the correlation of the received signal with a locally generated replica of the ranging code and detecting the highest peak or lowest valley.

The offset of the highest peak or lowest valley contains information about the code phase relative to receiver time.

If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers until a lock is obtained on one of the satellites.

The dot product is a complex number; its magnitude represents the level of similarity between the replica and the signal, as with an ordinary correlation of real-valued time series.

Likewise, as an improvement over the simple correlation method, it is possible to perform a single operation covering all code phases for each frequency bin.

The operation performed for each code phase bin involves forward FFT, element-wise multiplication in the frequency domain.

[11][12] There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data before computing the intersections of sphere surfaces.