GPS Data

The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
Data frames (1500 bits) are sent every thirty seconds. Each frame consists of five subframes.
Data bit subframes (300 bits transmitted over six seconds) contain parity bits that allow for data checking and limited error correction.

Clock data parameters describe the SV clock and its relationship to GPS time.
Ephemeris data parameters describe SV orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set.

Sample Ephemeris and Clock Data Parameters
SV Ephemeris Parameter to SV Position Algorithm
SV Clock Parameter to SV Clock Correction Algorithm

Almanacs are approximate orbital data parameters for all SVs. The ten-parameter almanacs describe SV orbits over extended periods of time (useful for months in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes (at least). Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanacs. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift caused by the rate of change in range to the moving SV) of each SV in the constellation.

Sample Almanac Parameters

Each complete SV data set includes an ionospheric model that is used in the receiver to approximate the phase delay through the ionosphere at any location and time.

Sample Ionospheric Parameters
Alpha[0] : 1.397E-08 Alpha[1] : 2.235E-08 Alpha[2] : -1.192E-07 Alpha[3] : -1.192E-07
Beta[0] : 1.044E+05 Beta[1] : 9.83E+04 Beta[2] : -1.966E+05 Beta[3] : -3.932E+05

Each SV sends the amount to which GPS Time is offset from Universal Coordinated Time. This correction can be used by the receiver to set UTC to within 100 ns.

Sample UTC Parameters
UTC
A0 : -9.3132E-09 sec A1 : -4.5297E-14 sec/sec dtLS : 1.0000E+01 sec tot : 4.6694E+05 sec
WNt : 2.9000E+01 weeks WNlsf : 2.4300E+02 weeks DN : 5.0000E+00 days
dtLSF : 1.0000E+01 sec

Other system parameters and flags are sent that characterize details of the system.

Position, and Time from GPS

Code Phase Tracking (Navigation)

The GPS receiver produces replicas of the C/A and/or P (Y)-Code. Each PRN code is a noise-like, but pre-determined, unique series of bits.
The receiver produces the C/A code sequence for a specific SV with some form of a C/A code generator. Modern receivers usually store a complete set of precomputed C/A code chips in memory, but a hardware, shift register, implementation can also be used.

C/A Code Generator

The C/A code generator produces a different 1023 chip sequence for each phase tap setting. In a shift register implementation the code chips are shifted in time by slewing the clock that controls the shift registers. In a memory lookup scheme the required code chips are retrieved from memory.

C/A Code Phase Assignments

The C/A code generator repeats the same 1023-chip PRN-code sequence every millisecond. PRN codes are defined for 32 satellite identification numbers.

C/A Code PRN Chips

The receiver slides a replica of the code in time until there is correlation with the SV code.

Correlation Animation
Short PRN Code Segment

If the receiver applies a different PRN code to an SV signal there is no correlation.

No PRN Correlation

When the receiver uses the same code as the SV and the codes begin to line up, some signal power is detected.

Partial PRN Correlation

As the SV and receiver codes line up completely, the spread-spectrum carrier signal is de-spread and full signal power is detected.

Full PRN Correlation

A GPS receiver uses the detected signal power in the correlated signal to align the C/A code in the receiver with the code in the SV signal. Usually a late version of the code is compared with an early version to insure that the correlation peak is tracked.

Simplified GPS Receiver Block Diagram

A phase locked loop that can lock to either a positive or negative half-cycle (a bi-phase lock loop) is used to demodulate the 50 HZ navigation message from the GPS carrier signal. The same loop can be used to measure and track the carrier frequency (Doppler shift) and by keeping track of the changes to the numerically controlled oscillator, carrier frequency phase can be tracked and measured.

Data Bit Demodulation and C/A Code Control

The receiver PRN code start position at the time of full correlation is the time of arrival (TOA) of the SV PRN at receiver. This TOA is a measure of the range to SV offset by the amount to which the receiver clock is offset from GPS time. This TOA is called the pseudo-range.
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