GPS Signals and Codes Overview

Study Notes

GPS satellites transmit on three frequencies: L1, L2, and L5.
The L1 signal is the oldest and most widely used signal. It is relatively slow and struggles to penetrate obstacles.
The L2 signal is more effective at traveling through obstacles like buildings and trees.
The L5 signal is the most advanced and offers improved accuracy and reliability.

GPS satellites transmit unique ranging code signals on L1 and L2 frequencies.
The Coarse Acquisition (C/A) code is transmitted on L1 and can be received by any GPS receiver. It transmits data at 1.023 million chips per second.
The Precision (P-code) is encrypted and available only to authorized users with specialized equipment. It transmits at 10.23 million chips per second.
Both codes are synchronized to the satellite's atomic clocks.

Satellites transmit a navigation message containing essential information:
- Ephemeris data: Provides information about the satellite's location, health, clock correction coefficients, and orbital parameters. It is used for real-time satellite coordinate calculation.
- Almanac data: Contains information about the health of all satellites, clock corrections, coarse orbital data, atmospheric delay parameters, and GPS time relative to UTC.

A GPS receiver calculates its distance from a satellite by measuring the time it takes for the satellite's signal to reach it. This assumes the receiver is on an imaginary sphere with the satellite at its center, approximately 20,000 kilometers away.
The receiver calculates distances to multiple satellites to determine its precise location through triangulation.

GPS satellites transmit three signals: L1, L2, and L5.
L1 is the oldest signal, and affordable devices can receive it. However, the relatively slow frequency makes it less effective at traveling through obstacles.
L2 was implemented after L1 and can effectively travel through obstacles.
L5 is the most advanced GNSS signal, offering a more accurate position.
L1 has a frequency of 1575.42 MHz and a wavelength of 190 mm.
L2 has a frequency of 1227.60 MHz and a wavelength of 244 mm.
L5 has a frequency of 1176.45 MHz.

Each GPS satellite transmits unique ranging codes on two frequencies: L1 and L2.
The Coarse Acquisition (C/A) code is transmitted on L1 and is accessible by any GPS receiver.
The Precision (P-code) code is transmitted on L1 and L2, and is encrypted. Only users with decryption equipment provided by the US Department of Defence can access it.
The C/A code transmits data at 1.023 million chips per second.
The P code transmits data at 10.23 million chips per second.
Both codes are synchronized to the satellite's atomic clocks.

Satellites transmit a navigation message that contains ephemeris data and almanac data.

Includes information on the week number, satellite accuracy and health, age of data, satellite clock correction coefficients, and orbital parameters.
This data is used for real-time satellite coordinate computation.

Contains an indicator of the health of all satellites, satellite clock corrections, coarse orbital data, atmospheric delay parameters, and the current GPS time and offset from UTC time.

A GPS receiver calculates its distance from a satellite by measuring the time it takes for a signal to reach it.
The distance is calculated by assuming the receiver is located on an imaginary sphere centered at the satellite (approximately 20,000 km in radius).
The receiver calculates the distance to multiple satellites to determine its own position.