What is the GPS L1 Band?

The GPS L1 band is the primary frequency band used by the Global Positioning Satellite constellation. It has a center frequency of 1575.42 MHz and a bandwidth of 15.345 MHz. The frequency range of the GPS L1 Band is 1563 to 1587 MHz. It is the most used GPS frequency band for navigation.

GPS L1 Band Signals

There are four different types of signals transmitted by the GPS on the L1 band: C/A, P (Y), M-code. and the recently introduced L1C signal.

C/A Code: The C/A or Coarse/Acquisition code is a type of civilian code that is designed for use in public, enabling navigation services for various users. It uses a unique Gold code, a binary sequence normally used in telecommunication and satellite navigation systems where each code combination generated by the satellite system is unique and therefore, helps multiple devices to transmit at the same frequency while avoiding interference due to neighboring users.

P (Y) Code: The P code is a precision signal and is reserved only for security applications such as military and defense. The term “Y” is a special code that may also be used when anti-spoofing capabilities are needed to further enhance security.

Both the C/A and P (Y) codes are transmitted at the same frequency while having different power levels to ensure that each can be differentiated from the other at the receiver. Furthermore, these two signals are transmitted at power levels below the thermal noise floor level, which makes them very difficult to be detected by unauthorized users. The figure below illustrates the power levels of the two codes.

M-Code: M-code is a signal used in the L1 band to improve security and anti-jamming capabilities of military navigation using GPS. M-code signals can be delivered to specific areas from GPS Block III satellites. It utilizes special modulation schemes to allow selective jamming of the commercial C/A code and continue receiving the desired M-code signals from the military receivers. In general, the M-code signal offers relatively better performance and flexibility compared to the P (Y) code.

L1C Signal: The L1C signal is a new signal type designed for broadcast at a higher power level. It uses two types of codes, which are then spread over a wide bandwidth using a ranging code and modulated by a data signal for transmission from the GPS satellite. An enhancement to the L1C signal, CHIMERA (chips message robust authentication) is also available. CHIMERA technique adds special encrypted information to the L1C signal that allows military forces to not only know when a signal is being spoofed but also makes it possible to authenticate the location of any GPS receiver.

Summary of GPS L1 Band Signals and their Properties

While these signals are used by all GPS systems, researchers are exploring other modulation schemes and signal processing/coding methods to further improve GPS navigation performance.

The GPS L1 band can also used together with the GPS L2 band to provide better accuracy. Dual-band receivers utilize signals from L1 and L2 frequency bands to minimize errors caused by ionospheric delays, atmospheric effects, and multipath interference to provide positional accuracy.

Click here to view all GPS frequency bands on everything RF.