New Civil Signals

A major focus of the GPS modernization program is the addition of new navigation signals to the satellite constellation.

The government is in the process of fielding three new signals designed for civilian use: L2C, L5, and L1C. The legacy civil signal, called L1 C/A or C/A at L1, will continue broadcasting in the future, for a total of four civil GPS signals. Users must upgrade their equipment to benefit from the new signals.

The new civil signals are phasing in incrementally as the Air Force launches new GPS satellites to replace older ones. Most of the new signals will be of limited use until they are broadcast from 18 to 24 satellites.

Second Civil Signal: L2C

L2C is the second civilian GPS signal, designed specifically to meet commercial needs.

Its name refers to the radio frequency used by the signal (1227 MHz, or L2) and the fact that it is for civilian use. There are also two military signals at the L2 frequency.

When combined with L1 C/A in a dual-frequency receiver, L2C enables ionospheric correction, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers enjoy the same accuracy as the military (or better).

GPS surveyor For professional users with existing dual-frequency operations, L2C delivers faster signal acquisition, enhanced reliability, and greater operating range.

L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors.

The Commerce Department estimates L2C could generate $5.8 billion in economic productivity benefits through the year 2030.

The first GPS IIR(M) satellite featuring L2C launched in 2005. Every GPS satellite fielded since then has included an L2C transmitter.
Learn more about GPS IIR(M)

On April 28, 2014, the Air Force began broadcasting civil navigation (CNAV) messages on the L2C and L5 signals. Prior to that, L2C and L5 provided a default message (Message Type 0) containing no data. Adding the CNAV capability required upgrades to the GPS control segment.
Learn more at defense.gov

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Third Civil Signal: L5

L5 is the third civilian GPS signal, designed to meet demanding requirements for safety-of-life transportation and other high-performance applications.

Its name refers to the U.S. designation for the radio frequency used by the signal (1176 MHz).

L5 is broadcast in a radio band reserved exclusively for aviation safety services. It features higher power, greater bandwidth, and an advanced signal design.

Airplane flying overhead Future aircraft will use L5 in combination with L1 C/A to improve accuracy (via ionospheric correction) and robustness (via signal redundancy).

In addition to enhancing safety, L5 use will increase capacity and fuel efficiency within U.S. airspace, railroads, waterways, and highways.

Beyond transportation, L5 will provide users worldwide with the most advanced civilian GPS signal. When used in combination with L1 C/A and L2C, L5 will provide a highly robust service. Through a technique called trilaning, the use of three GPS frequencies may enable sub-meter accuracy without augmentations, and very long range operations with augmentations.

In 2009, the Air Force successfully broadcast an experimental L5 signal on the GPS IIR-20(M) satellite. The first GPS IIF satellite with a full L5 transmitter launched in May 2010. Like L2C, the L5 broadcast will not include a data message until the GPS control segment is upgraded.
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On April 28, 2014, the Air Force began broadcasting civil navigation (CNAV) messages on the L2C and L5 signals. Prior to that, L2C and L5 provided a default message (Message Type 0) containing no data. Adding the CNAV capability required upgrades to the GPS control segment.
Learn more at defense.gov

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Fourth Civil Signal: L1C

L1C is the fourth civilian GPS signal, designed to enable interoperability between GPS and international satellite navigation systems.

Its name refers to the radio frequency used by the signal (1575 MHz, or L1) and the fact that it is for civilian use. There are also two military signals at L1, as well as the legacy C/A signal. L1C should not be confused with L1 C/A.

Tokyo city street L1C features a Multiplexed Binary Offset Carrier (MBOC) modulation scheme that enables international cooperation while protecting U.S. national security interests. The design will improve mobile GPS reception in cities and other challenging environments.

The United States and Europe originally developed L1C as a common civil signal for GPS and Galileo. Other satellite navigation providers are adopting L1C as a future standard for international interoperability. Japan's Quasi-Zenith Satellite System (QZSS), the Indian Regional Navigation Satellite System (IRNSS), and China's Compass system all plan to broadcast L1C.
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The United States will launch its first L1C signal with GPS IIIA. L1C will broadcast at the same frequency as the original L1 C/A signal, which will be retained for backwards compatibility.

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Semi-Codeless/Codeless Transition Plan

Once L2C and L5 are fully operational, their features will obviate the need for codeless or semi-codeless GPS receivers, which many GPS professionals use today to attain very high accuracy. Such receivers work by exploiting characteristics of the encrypted military P(Y) signal at the L2 frequency to achieve dual-frequency capability.

The U.S. government encourages all users of codeless/semi-codeless GPS technology to plan on using the modernized civil signals by December 31, 2020, as P(Y) may change after that date.
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