GPSsignal

GPSsignal is the radio frequency component of the Global Positioning System used by GPS receivers to determine position, velocity, and time anywhere on Earth. GPS satellites broadcast multiple spread-spectrum signals on L-band frequencies. Each satellite uses a unique pseudorandom code that allows receivers to identify satellites and to align their internal clocks with satellite clocks, enabling precise timing measurements.

The most widely used civilian signal is the L1 C/A signal at 1575.42 MHz. It carries a 50-bit-per-second navigation data stream, with navigation messages organized in frames that repeat every 30 seconds. The data include satellite ephemeris, almanac, and clock corrections. Receivers compute pseudorange by measuring code phase of the C/A code and the carrier Doppler, then combine measurements from at least four satellites to solve for position and receiver clock bias.

In addition to the C/A signal, GPS transmits encrypted P(Y) signals for military use on L-band, and newer civil signals such as L2C and L5 that provide improved robustness, higher accuracy, and better performance in difficult conditions.

Reception and processing: the receiver tracks the PRN code sequences, decodes the navigation data, estimates code phase, carrier phase, and Doppler, and uses the ephemeris to calculate the satellite positions. The result is a set of range measurements that are solve for a 3D position and time.

Corrections and augmentation: ionospheric delay, multipath, and atmospheric effects degrade accuracy. Differential GPS, SBAS (such as WAAS or EGNOS), and precise orbit and clock corrections improve accuracy and reliability, enabling applications from navigation to surveying and timing.