Navigation Services

GPS Policy - Modernization

Policy

• PNT
• National PNT Engineering Forum
• Joint Program Office
• GPS Modernization
• GPS Evolutionary Architecture Study (GEAS)
• Selective Availability
• Presidential Policy

The Future of Navigation

For several years now, the Global Positioning System (GPS) has been recognized as the future of navigation for most military and civil applications, including those for various modes of transportation. The civil aviation community has been one of the main benefactors of GPS due to its flexibility and worldwide applicability. In 1999, as a direct result of these benefits to the civil community, Vice President Al Gore announced that the U.S. would embark on a GPS modernization effort to extend the capabilities of GPS even further than those of the existing GPS constellation.

GPS Modernization

One of the main components of this modernization is the addition of two new navigation signals for civil use. These signals will be in addition to the existing civilian service broadcast at 1575.42 MHz (L1). The first of these new signals will be a new civil code, called L2C, which will be added on the existing L2 carrier, located at 1227.60 MHz.  It will be available for general use in non-safety critical applications. The Block IIR-M satellite, the first to add his capability was launched September 25, 2005.
A third civil signal, located at 1176.45 MHz (L5), will be provided initially on GPS Block IIF satellites beginning in 2007, and continuing with the Block III satellites scheduled for launch beginning in 2012. This new L5 signal is protected worldwide for aeronautical radionavigation use, and will support aviation safety-of-life applications. The addition of L5 will make GPS a more robust radionavigation service for many aviation applications, as well as all ground-based users (maritime, railways, surface, shipping, agriculture, recreation, etc.)
At the current GPS satellite replenishment rate, all three civil signals (L1-C/A, L2C, and L5) will be available for initial operational capability by 2012, and for full operational capability by approximately 2015.

Benefits of L5

L5 will provide significant benefits above and beyond the capabilities of the current GPS constellation, even after the planned second civil frequency (L2) becomes available. Benefits include precision approach navigation worldwide, increased availability of precision navigation operations in certain areas of the world, and improved interference mitigation.

Precision Navigation Operations Worldwide

GPS offers the capability to provide a safe and efficient seamless satellite-based global navigation system that will be responsive to the future needs of civil aviation. The addition of L5 will provide aviation and other transportation applications with continuous, highly accurate, three-dimensional position information.
The availability of L5 offers the potential of providing precision approach capability throughout the footprint of a satellite-based augmentation system (SBAS) geostationary (GEO) satellite. Even outside the defined SBAS service areas, users in the footprint of these broadcasts, which is essentially most of the world, will be able to use dual frequency avionics (L1 and L5) to provide a precision approach capability with little or no ground infrastructure investment. These dual frequency avionics will use integrity data from augmentation system corrections that are broadcast from Geostationary Earth Orbit Satellites (GEOs) on L1 and L5.
In the event of interference on either L1 or L5, these regions within the footprints, can revert to non-precision approaches using GPS with integrity data from SBAS corrections broadcast on L1 or L5 or GPS with receiver autonomous integrity monitoring (RAIM).

Increased Availability of Precision Navigation Operations

GPS modernization is compatible with and complements the WAAS. The L5 signal will be available on additional WAAS GEOs launched in September and October 2005. When both L1 and L5 are available, avionics will use a combination of signals to provide the most accurate service possible, thereby increasing availability of the service. These avionics will use ionospheric corrections broadcast by WAAS, or self-generated onboard dual frequency corrections, depending on which one is more accurate.

Interference Mitigation

Due to the low power of GPS signals, interference has been identified as a challenge related to the use of GPS. Despite this, the technical feasibility of using augmented GPS as the only means of navigation in the aircraft as well as the only navigation service provided by the Federal Aviation Administration (FAA) was confirmed as technically feasible in an independent study conducted by Johns Hopkins University Applied Physics Laboratory. However, the study did recommend efforts to mitigate the effects of intentional interference, unintentional interference, and atmospheric disturbances on GPS and its augmentation systems. The mitigation of interference is not only important to aviation, but to other GPS applications as well. To mitigate the effects of interference and any impact on civil aviation safety-of-life operations, the FAA is planning to use the new navigation signal at L5 (1176.45 MHz) for both WAAS and the Local Area Augmentation System (LAAS).
The availability of L5 will encourage the development of avionics that can use GPS signals on both L1 and L5 to eliminate ionospheric errors. For example, WAAS will broadcast ionospheric corrections and integrity data on both L1 and L5. In the event of interference on either L1 or L5, avionics will be capable of conducting precision approaches, as currently designed by the FAA, using the GPS and the WAAS ionospheric corrections and integrity data broadcast on whichever frequency is available (either L1 or L5).
Similarly, LAAS will broadcast corrections and integrity data pertinent to both L1 and L5. In the event of interference on either L1 or L5, the aircraft will be able to conduct precision approaches using the data broadcast for the frequency available. The addition of L5 will have a profound impact on both the new and existing applications of GPS for navigation.
For more information on GPS Modernization Activities, please visit the GPS Wing website.